Change some warnings in utf8n_to_uvchr()

This bottom level function decodes the first character of a UTF-8 string
into a code point.  It is discouraged from using it directly.  This
commit cleans up some of the warnings it can raise.  Now, tests for
malformations are done before any tests for other potential issues.  One
of those issues involves code points so large that they have never
appeared in any official standard (the current standard has scaled back
the highest acceptable code point from earlier versions).  It is
possible (though not done in CPAN) to warn and/or forbid these code
points, while accepting smaller code points that are still above the
legal Unicode maximum.  The warning message for this now includes the
code point if representable on the machine.  Previously it always
displayed raw bytes, which is what it still does for non-representable
code points.