Add on_error handler

[towhans]

This allows handling cases when Hammer Backend returns {:error, ...} tuple. For example Hammer.Backend.Redis can return {:error, %Redix.ConnectionError{}} which is not technically an over limit return value but more of a Internal Server Error response.

This will allow to return 500 https status code in case Hammer.Backend returns an {:error, _} which is now impossible because the on_deny handler is called both for :deny and :error The new optional handler can handle error case.