Ruby custom error classes: inheritance of the message attribute

raise already sets the message so you don’t have to pass it to the constructor:

class MyCustomError < StandardError
  attr_reader :object

  def initialize(object)
    @object = object
  end
end

begin
  raise MyCustomError.new("an object"), "a message"
rescue MyCustomError => e
  puts e.message # => "a message"
  puts e.object # => "an object"
end

I’ve replaced rescue Exception with rescue MyCustomError, see Why is it a bad style to `rescue Exception => e` in Ruby?.