The code does not expect sh_error() to return, but the patch
that made this header usable in frontend didn't get that memo.

While here, plaster unlikely() on the tests that decide whether
to invoke sh_error(), and add our standard copyright notice.

Noted by Andres Freund.  Back-patch to v13 where this frontend
support came in.

Discussion: https://postgr.es/m/0D54435C-1199-4361-9D74-2FBDCF8EA164@anarazel.de