Is best_match() well defined?

[Julian]

Given an iterable of errors, can we define a useful best_match function that returns the best match of the given errors (i.e. the error that seems to most reasonably represent the validation failure?)

Rough idea:

• Weigh how deep path and schema_path go into the instance / schema.
• Use __lt__ on _Error ( 🎀 ) and return max(errors). Why? Because we can.