Adding support for onerror parameter in walk

[epizut]

Fixes #1297, I was not sure how to test the async version since I haven't found any suitable existing test to start with.

The current walk() implementation is missing the onerror parameter.

See os.walk

[martindurant]

I quite like onerror being a function, but in the existing API, it's a string like "omit", "raise"... Can you please revisit other usage and try to achieve consistency?

For async, the tricky part can be to figure out when to consider errors. Most of the coroutines will return error instances, so we can check for that. You will need to look over in s3fs for probably the most complete application.

[epizut]

Sorry, I miss the existing on_error, I planned to use a function to mimic os.walk().
I defaulted on omit to avoid any behavior change, this differs from the existing on_error usage that raises by default. Unfortunately, find() expects walk() to not raise when called with a file path. I will be happy to default on raise, but it will need a few more changes.