Don't use EntityIdParser if only ItemIds are supported

Also improved some error messages.

Change-Id: Ie6cc3dd47fca2423d33acc258f5eadeb117dcfea
Replace deprecated getPrefixedId in Client and Lib

In case you wonder, the removed method is unused and broken anyway.

Change-Id: I6fa896855890d4195663a946a9a67356ee47a3f6
Merge "Don't use EntityIdParser if only ItemIds are supported"