Handle NotFound exception in snapshots API code

Passing tests in the gate leave unhandled trace/error messages
for a VolumeNotFound issue.

These are caused by the test_volumes_snapshots_negative test
which requests a non-existent volume and the tests pass however
the log files are a bit messed up due to the unhandled exception.

This change just adds a try/catch block around the volume get calls
in the snapshot modules and raises HTTPNotFound as appropriate.

Change-Id: I2096f2da7c68ef7924fc8e69b2d5c2afea578512
Closes-Bug: #1255214
Merge "Handle NotFound exception in snapshots API code"