Throw StorageException in EntityTermLookup for non-existing entities

consistent with the EntityRetrievingTermLookup and to
allow different handling of the case of Entity does
not exist vs. Entity exists but term not found.

also updated documentation of LabelLookup.

Change-Id: I63ccb80447a19396cde607308ef75d38b4234861
Move wikibase.utilities from lib to repo

This is currently only used by jquery.wikibase.*, which will soon be moved to
repo, too.

Change-Id: Ibc9f7d56e07ed5016fc63712f052684247fe36fb
Merge "Throw StorageException in EntityTermLookup for non-existing entities"
Merge "Move wikibase.utilities from lib to repo"