exception_notification エラー ref: http://qiita.com/shu_0115/items/2ef1c6ae01bccc8b0203

file0.txt

/Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/actionpack-4.0.0.rc2/lib/action_dispatch/middleware/stack.rb:43:in `build': undefined method `new' for ExceptionNotifier:Module (NoMethodError)
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/actionpack-4.0.0.rc2/lib/action_dispatch/middleware/stack.rb:118:in `block in build'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/actionpack-4.0.0.rc2/lib/action_dispatch/middleware/stack.rb:118:in `each'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/actionpack-4.0.0.rc2/lib/action_dispatch/middleware/stack.rb:118:in `inject'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/actionpack-4.0.0.rc2/lib/action_dispatch/middleware/stack.rb:118:in `build'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/engine.rb:495:in `app'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/application/finisher.rb:34:in `block in <module:Finisher>'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/initializable.rb:30:in `instance_exec'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/initializable.rb:30:in `run'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/initializable.rb:55:in `block in run_initializers'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:150:in `block in tsort_each'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:183:in `block (2 levels) in each_strongly_connected_component'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:219:in `each_strongly_connected_component_from'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:182:in `block in each_strongly_connected_component'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:180:in `each'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:180:in `each_strongly_connected_component'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/2.0.0/tsort.rb:148:in `tsort_each'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/initializable.rb:54:in `run_initializers'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/application.rb:215:in `initialize!'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/railtie/configurable.rb:30:in `method_missing'
    from /Users/shu/labo/cakes-on-web/config/environment.rb:5:in `<top (required)>'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/backports-3.3.2/lib/backports/tools.rb:328:in `require'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/backports-3.3.2/lib/backports/tools.rb:328:in `require_with_backports'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/activesupport-4.0.0.rc2/lib/active_support/dependencies.rb:228:in `block in require'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/activesupport-4.0.0.rc2/lib/active_support/dependencies.rb:213:in `load_dependency'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/activesupport-4.0.0.rc2/lib/active_support/dependencies.rb:228:in `require'
    from /Users/shu/labo/cakes-on-web/config.ru:3:in `block in <main>'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/builder.rb:55:in `instance_eval'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/builder.rb:55:in `initialize'
    from /Users/shu/labo/cakes-on-web/config.ru:in `new'
    from /Users/shu/labo/cakes-on-web/config.ru:in `<main>'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/builder.rb:49:in `eval'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/builder.rb:49:in `new_from_string'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/builder.rb:40:in `parse_file'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/server.rb:277:in `build_app_and_options_from_config'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/server.rb:199:in `app'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/commands/server.rb:48:in `app'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/server.rb:314:in `wrapped_app'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/commands/server.rb:75:in `start'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/commands.rb:80:in `block in <top (required)>'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/commands.rb:75:in `tap'
    from /Users/shu/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/railties-4.0.0.rc2/lib/rails/commands.rb:75:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'

file1.txt

gem 'exception_notification', github: 'smartinez87/exception_notification'

file2.txt

gem "exception_notification", "~> 4.0.0.rc1"

file3.txt

CakesOnWeb::Application.config.middleware.use(
  ExceptionNotifier,
  email_prefix:         "[Whatever] ",
  sender_address:       %{"notifier" <notifier@example.com>},
  exception_recipients: %w{exceptions@example.com},
)

file4.txt

CakesOnWeb::Application.config.middleware.use ExceptionNotification::Rack,
  email: {
    email_prefix:         "[Whatever] ",
    sender_address:       %{"notifier" <notifier@example.com>},
    exception_recipients: %w{exceptions@example.com}
  }

Did you figure out the problem?

Read the part on the home page where it says "Upgrading to 4.x version"

Especially this line: Beside that, the rack middleware was renamed to ExceptionNotification::Rack

It solved my issue.