Move exception definitions out of db api

The two exceptions, NoMoreNetworks and NoMoreTargets, are defined
in nova/db/api. While NoMoreTargets is unused and can be removed,
NoMoreNetworks are still used and should be moved to exception.py
where all other exceptions live.

This came up during review of https://review.openstack.org/#/c/42980/
where compute manager would need to import nova.db to make use of
NoMoreNetworks.

Change-Id: I57f0a081d2fea35394421932ae7d2a3e67540a31