Move WikibaseExceptionLocalizer messages to lib

$message->exists() in WikibaseExceptionLocalizerTest does not
work when running tests in the client while the messages are
in repo.

Change-Id: Ib55d3430cc572b727c0e7621650de8ea4aa2b5a4
(cherry picked from commit aa7c85dd106563c5d17fbe5071fee37c92b8c350)