roomservice.py: Verbose error when a branch is missing.
Also add ROOMSERVICE_BRANCHES environment variable to use fallback branches.

Change-Id: I3c2b1d79fc185c1f1e1d658e5ca4f78e688780e2
roomservice.py: Fixups around fallback branches not being used by dependencies when ROOMSERVICE_BRANCHES is defined.

Change-Id: Ifb42a023cae5f62ac8f9cf7832125b91b431169c
roomservice: Allow following up tag references

This is now needed for release builds

Change-Id: I8c5f87341059b3b15ee853312b71df73790ad0d8