Resource.find: allow returning empty instead of raising

Add flag raise_if_missing, and if False, instead of raising an exception,
return an empty list for that kind of resource. This makes sense for a caller
that requests a single resource.

When finding a single resource to use in ReservedResources.get(), use this to
raise a more adequate exception message if none was found.

Change-Id: Ia296ea68a787bede037a6cea38563b570fb0766e
1 file changed