locale.c: Clarify warning message

locale.c: Clarify warning message

When there are discrepancies in the locale and what Perl is expecting, a
warning is raised listing the problematic characters.  For \n, and \t,
they should have been displayed as mnemonics, but a required backslash
to escape things had been omitted, so they were displayed literally, so
looked just like white space.  Also, put any displayed blank in ' ' so
it won't look like the list is empty.