If _UnlockedLookupNetwork() fails raise error

Make _UnlockedLookupNetwork() raise OpPrereqError (instead of returning
None) in case it does not find the requested network. Remove useless and
duplicate code such as:

if net_uuid is None:
  raise...

This is a cherry-pick of commit 1cce2c47.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>