Have a parameter to supply a proc that is only called on the final failure

[jcobian]

Currently the on_retry parameter is called each time the passed in block of code raises a matching exception.

I am proposing that there be a on_final_failure (or similarly named) parameter which is a proc that is called after all retries have been exhausted.

The use case is that:

• sometimes I'd like to perform a different action once all the retries have finished
• sometimes I only want to perform an action once all the retries are finished.

I know there are current ways to do this outside of the gem, but I think the following interface could be nice:

do_this_on_each_retry = Proc.new { }
do_this_on_final_retry = Proc.new { }
Retriable.retriable(on_retry: do_this_on_each_retry, on_final_failure: do_this_on_final_retry) do
  # code here..
end

This allows me to have custom logic for the final failure, while still allowing the error to be raised by Retriable