FR: a way to return an error that should not be retried

[ajeetdsouza]

• In cenkalti/backoff, it is implemented using a PermanentError type.
• In avast/retry-go, it is implemented using the RetryIf function and the Unrecoverable type.

[tzachshabtay]

Note that you can do that with reinforcer (a utility that generates go-resilience code), the readme has an example for exactly that.