Add in exceptions and verbose errors

[muellerzr]

Includes a variety of verbose errors for commonly situated problems. Due to the complexity of some issues, some of the logic in said errors is a bit much in order to provide a clear picture of what went wrong (such as the collate_error).

Open to removing any you're not comfortable with @jph00

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[muellerzr]

Aware there's a small bug with the tests in there I think. Will get to it in the next few days

[jph00]

@muellerzr can you take a look at this CI failure? Might just be a case of needing to merge lastest fastai changes - not sure.

[muellerzr]

@jph00 okay, that was an exploration and a half but we got callbacks working. One thing I did have to do though was move up where the exception classes were defined so they can be pulled into Callback.__call__. Since we already use show_doc for them all though, I think it would make sense to keep the documentation in the same spot

[muellerzr]

We could also rewrite this as something like:

except Exception as e:
  if len(e.args) > 0: 
    e.args = [f'Exception occured in `{self.__class__.__name__}` when calling event `{event_name}`:\n\t{e.args[0]}']
  raise

However that could potentially lead to trouble down the road. (I could also do a huge isinstance too)

[jph00]

Thanks @muellerzr !