setup for @app.call errors

gistfile1.rb

builder = Rack::Builder.new do
  use Rack::CommonLogger
  use Rack::Lint
  use Rack::ShowExceptions
  lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['OK']] }
  use Rack::Lint
end
@app = builder.to_app

# pass app to server, server calls app and gets response

status, headers, body = @app.call(env)

# ^ will be one of the used Rack:: classes, not 200