Gracefully handle fetching nonexistent rule

Gracefully handle fetching nonexistent rule

Currently, if we invoke the API for 'show rule' but the rule does not
exist, an exception is raised from deep within Neutron. This in turns
causes an uncaught exception and the user will see 'ServerFault'. This
patch proposes a fix for this scenario - the case where the rule does
not exist is handled and a NeutronException is caused, causing a proper
'NotFound' error on the client side instead.

Partially-Implements: blueprint quantum-qos-api
Change-Id: Ic703a0865d1cfa057ab1ad5290b793b22df06af6