Get should throw exception if not unique
I've commented already that the semantics of get() are not going to change, so I will pass on this change.
 3.0 man, 3.0!  :-)
 It's not changing in 3.0 either man.

again, behavior changing, so not for any point release, but hopefully some future incantation.
re: https://github.com/coleifer/peewee/issues/958
