exception: remove

This patch drops the obsolete openstack.common.exception module.

Most project should define their own exception based on their API and
the context they evolve in. Many projects don't use this, only neutron,
cinder and heat used it. I've copy pasted the few exceptions that they
were using, but that's still by far less code than this whole file.
Ultimately that will let them a chance to remove or shring their own
exception.py. I don't think most projects should have one anyway.

Change-Id: Ia8b2b29bd443233def324e97de7342c2634bccff
Closes-Bug: #1208734