Use error type instead of using catch all failure::Error in Producer::con

[wezm]

Per the failure guidance the failure::Error type is typically only recommended when prototyping or for "applications" that won't handle the error. It would be better to define a custom error type and 'derive Fail' for it.

I see there are some parts of the code using a custom error type but others are falling back on Error.

If this sounds reasonable I'm happy to make this change.

[jonhoo]

For the various methods on Producer, I completely agree that a custom error type would be useful. For Consumer::run* it's less clear that the user of the library can do anything useful on error, so I'm less concerned. Happy to take a look at a PR!

[jonhoo]

@wezm still want to take a stab at this too? I'd be happy to review and merge!

[wezm]

I've moved away from using Faktory for my project so don't have a huge personal need to implement this or #9 (comment). However I do need a couple more PRs for Hacktoberfest so might see if I can implement this one.