Raise HTTPNotFound in V2 console extension

In case requested instance is not found, console extension does not catch
InstanceNotFound, which results to internal server error (500).

This patch handle InstanceNotFound exception and raise HTTPNotFound
like V3 console plugin.

Change-Id: Ia982049451f2c529f123da714e994476d4610491
Merge "Raise HTTPNotFound in V2 console extension"