rest: add_measures never raises MetricDoesNotExist
rest: add_measures never raises MetricDoesNotExist

This is not true anymore since Gnocchi has been made asynchronous for
its measures processing.

Change-Id: I63967cfc4a58dad5630cda1d6dbd43a6d31291d1

gnocchixyz/gnocchi,gnocchixyz/gnocchi,sileht/gnocchi,leandroreox/gnocchi,leandroreox/gnocchi,sileht/gnocchi