Output appropriately dire warning for high code points

Output appropriately dire warning for high code points

Code points above 2**31-1 have never been in any standard, so their use
is non-portable.  There has been a more dire warning raised at times
when decoding UTF-8 into a UV, but it wasn't getting output always when
it was appropriate.  Going the other way, into UTF-8, there was only one
type of warning output.  This commit splits that into the more dire for
higher code points.