Exceptions raised by quantum validate_networks result in 500 error

The exceptions raised by validate_networks should be subclassed
from Invalid so they result in a 400 error

These exceptions are also used in allocate_for_instance

Fixes bug 1193421

Change-Id: If708e413d81a9d32bf81e7821d876736c4ac40ef
Merge "Exceptions raised by quantum validate_networks result in 500 error"