Fix frontend version of sh_error() in simplehash.h.
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