Export 'client' and 'exc' modules

To facilitate cleaner importing of this package,
explicitly export the 'client' and 'exc' modules. Also add
in an exported alias for 'exc' called 'exceptions' to help
migrate away from the user-unfriendly 'exc' name.

Co-authored-by: Michael Davies <michael@the-davies.net>

Change-Id: I59ea5c439dbf6ff30cc65cf95491b47bbbd0c516