Give proper error for `\x{' regex, alter a few messages

Instead of a low-level error when a trailing `\x{' construction
is not closed we now have a proper error, as tested by the new
m3regex009.  Also add a few quotes to some messages and fix a
singular/plural problem.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5061 de43f980-851b-0410-b2f7-c40aca1f87e0