patch.rb

begin
  require 'byebug/core'
rescue LoadError
  return
end

module Byebug
  def self.byebug_on_error
    unless started?
      self.mode = :attached
      start
      run_init_script
    end
    old_value = post_mortem?
    self.post_mortem = true
    begin
      yield
    rescue Exception => ex
      ContinueCommand.allow_in_post_mortem = true
      Byebug::Context.interface.errmsg("#{ex.class.name}: #{ex.message}")
      handle_post_mortem
      ContinueCommand.allow_in_post_mortem = false
      raise ex
    ensure
      self.post_mortem = old_value
      stop if stoppable?
    end
  end
end

test.rb

require 'byebug'
require_relative './patch'

def test_call
  foo = 1
  fooo += 1
end

# Run the bad function, triggering a post-mortem debugging session on error:
Byebug.byebug_on_error do
  test_call
end

# The reason for re-raising the exception is to enable error recovery:
begin
  Byebug.byebug_on_error do
    test_call
  end
rescue
  # Log error, continue process
end