Raise exceptions from baremetal/db/sqlalchemy/api.py
following the precedents from nova/db/sqlalchemy/api.py.
Fixes tests to expect the new exceptions.
Adds __init__ so that baremetal tests work in isolation.
blueprint general-bare-metal-provisioning-framework
Change-Id: Ic8ef66e8f3180460a7bf117fccbfe15078905d8b