Add optional retry_error_callback param

[gulducat]

Being able to call a custom function when all retries have failed (without necessarily raising an exception) is useful for a few applications.

• Do nothing (raise no exception without repeated try/except boilerplate or extra function wrapper)
• Return a default value
• Return the value of the last attempt (if retry_if_result for example)
• Really anything you can think to do with the last Future object

A couple of these uses were issues on the retrying module, which I made a PR to (rholder/retrying#78) before realizing this is probably a better place for it.

[gulducat]

Hey there @jd - looks like you addressed the mergify failure in 1c0b572 :)

Any feedback on this PR that I can address? (aside from merging in latest master)

[jd]

@dbwpe Looks good to me, released 4.12.0 with that.