Allow packages to provide custom hints for Exceptions (#35094)
Package authors may be able to predict likely user errors, and it can
be nice to create an informative hint. This PR makes it possible for packages to register
hint-handlers for a variety of error types via `register_error_hint`. For packages that
create their own custom Exception types, there is also `show_error_hints` which may
be called from the `showerror` method.
