I’m writing a library to extend a Ruby gem, specifically selenium-webdriver. It involves subclassing the Firefox::Driver class.

In my subclass when should I define/use custom error classes and when should I use the error classes already available to the superclass? E.g.

require 'selenium-webdriver'

module Selenium
  module WebDriver
    module Firefox
      class SpecializedDriver < Driver
        class SpecializedError < Error; end # when to do this?

        def initialize; end

        def foo
          raise SpecializedError if some_condition # or e.g. Selenium::WebDriver::Error
        end
      end
    end
  end
end

Similarly, with new modules when should I use a custom error class IFO the library’s existing error classes?

A similar question was asked here as a supplementary question, but AFAICT it has never actually been answered.

Are there any conventions that establish when to and when not to
create a new error type?