Add utf8n_to_uvchr_error

Add utf8n_to_uvchr_error

This new function behaves like utf8n_to_uvchr(), but takes an extra
parameter that points to a U32 which will be set to 0 if no errors are
found; otherwise each error found will set a bit in it.  This can be
used by the caller to figure out precisely what the error(s) is/are.
Previously, one would have to capture and parse the warning/error
messages raised.   This can be used, for example, to customize the
messages to the expected end-user's knowledge level.