qos-engine: prevent NPE if pool monitor not available

[alrossi]

trafficstars

Motivation:

Apparent race in system tests where qos-engine does not receive the pool monitor message in time to handle incoming cache location messages.

Modification:

Avoid reporting an NPE by throwing an explicit exception when pool monitor is accessed but is null.

Result:

No runtime stack trace.

Target: master Request: 8.2 Request: 8.1 Requires-notes: no (discovered via system test) Requires-book: no Bug: #6777 Closes: #6777 Acked-by: Tigran