Access all occurred exceptions in RetryError

[mtvx]

It would be useful to have access to the exceptions/stacktraces of all attempts, via RetryError.

This would allow to create e.g. metrics out of failed attempts and in general have some logic based on all failures (not only the last).

[blr246]

I have a way to do this that I was just thinking would be good to contribute upstream. It uses the existing API so it's really just like a wrapper / extension. I'm still looking for the contribution guidelines for this project. I'll check in again during the workweek with more info.