Allow custom error handling

[gwauge]

It would be great, if the error/retrying logic was customizable.

Specifically, we need to retry on more than just the specified error types, for example OSError.
Additionally, on ClientErrors s3fs currently only retries if one of a few specific patterns matches. Being able to modify this behavior helps with handling non-standard error messages from object storage providers, such as retrying on "please throttle!".

This PR adds global methods add_retryable_error & set_custom_error_handler, which allow users to customize this behavior.

[martindurant]

Thanks, this is a good idea!
There is quite a lot here, so it will take me some time to review.

[martindurant]

Everything looks good herem, I am happy to merge as-is, and I would like to get it into the release that I will likely do later today.

One thing you might want to change: it isn't clear in the docs (to me) that the custom error handler applied a condition in addition to the standard ones that were there before: so if your exception matches something retriable, there's no way with this mechanism to not retry it. I can imagine a follow-up to this that gives full flexibility.