Retry error message

[lucastoro]

I propose a small change to improve the meaningfulness of the string representation of a (failed) Attempt. Now when serializing the Attempt instance (likely when catching a RetryError exception) the returned string contains a customizable message or, when not specified, the name of the function that was wrapped by @retry. I updated the unit tests as well to verify the correct behavior of the feature and of course checked that other functionalities were not affected.

P.s. I know I should have opened an issue first, but I thought it would be faster to just make a PR.