customizable augment_usage_errors

I was looking for a global and clean way to process exception raised inside the command callback. Something like flask.errorhandler.

I read a lot of click code and actually it is not easy to customize because it is all built to be use with decorator.

Finally I found augment_usage_errors, a contextmanager used to wrap the command callback.
I think that will be nice to have a way to customize the exception handling and maybe augment_usage_errors is the starting point to do that.
I'm not so confident with the click code thus I'll not propose a PR, but I would hear your comments about this.

Anyway my needs was really stupid: I want to print the exception stack-trace if I am in debug mode otherwise I'll print a coloured one-line message.

Start by reading Click's augment_usage_errors context manager and the command callback exception path. Compare the requested global handling with Flask's errorhandler model and determine the intended customization boundary. Done means the issue's debug and one-line colored error-handling needs have a clear, maintainable extension point, with its behavior documented and tested.