This helps to implement CommandError::add_hint(). The inner errors could be
embedded in the enum as before, but they're mostly of the same type. And I think
it's okay to use downcast_ref() to deal with the clap::Error special case.
I'm going to reorganize CommandError as (kind, err, hints) tuple so that we
can add_hint() to the constructed error object.
Some config error messages are slightly adjusted because the inner error is
now printed in separate line.
This should address both use cases:
1. If from_relative_path() is directly called, the error says ".." shouldn't
be included in the (normalized) relative path.
2. If parse_fs_path() is used, the error message contains paths relative to
cwd. #3216
