Auto-retry declared classes of errors

[ryanwi]

For some errors, like timeouts, retries may be safe enough to auto-retry.

[semmons99]

We might also want to auto-purge certain errors like done here in a resque-check job I wrote: https://gist.github.com/semmons99/9e2dd789dd7809e4c5df

[knappe]

We have a partial solution in place. We have a config option that will allow you to retry specified types of errors, but we don't yet have a way also discard errors.