Add option to specify a callback to be called on failure.

[mboyanov]

Add option to specify a callback to be called on failure. This comes in handy if you want to make a health check or reset an external service or refresh a token.

[nilansaha]

This is extremely useful. Unfortunately this repo has been inactive for a while. Is the owner even around ?

[leshchenko1979]

Merged this into https://github.com/leshchenko1979/reretry