Partial resolution to bug 121783

This change replaces RuntimeError exceptions in sqlalchemy/api.py
with NotFoundException which is derived from SavannaException. The
use of SavannaException will cause errors to be reported with more
detail through the REST api.

Change-Id: I570d5f4b0f07a3f0406180074e413a47cee51dd7
Implements: 121783