Improve our response to invalid format strings, and detect more cases.

Places that are testing for *printf failure ought to include the format
string in their error reports, since bad-format-string is one of the
more likely causes of such failure.  This both makes it easier to find
and repair the mistake, and provides at least some useful info to the
user who stumbles across such a problem.

Also, tighten snprintf.c to report EINVAL for an invalid flag or
final character in a format %-spec (including the case where the
%-spec is missing a final character altogether).  This seems like
better project policy, and it also allows removing an instruction
or two from the hot code path.

Back-patch the error reporting change in pvsnprintf, since it should be
harmless and may be helpful; but not the snprintf.c change.

Per discussion of bug #15511 from Ertuğrul Kahveci, which reported an
invalid translated format string.  These changes don't fix that error,
but they should improve matters next time we make such a mistake.

Discussion: https://postgr.es/m/15511-1d8b6a0bc874112f@postgresql.org

src/common/psprintf.c

@@ -129,9 +129,9 @@ pvsnprintf(char *buf, size_t len, const char *fmt, va_list args)
 	if (nprinted < 0 && errno != 0 && errno != ENOMEM)
 	{
 #ifndef FRONTEND
-		elog(ERROR, "vsnprintf failed: %m");
+		elog(ERROR, "vsnprintf failed: %m with format string \"%s\"", fmt);
 #else
-		fprintf(stderr, "vsnprintf failed: %s\n", strerror(errno));
+		fprintf(stderr, "vsnprintf failed: %m with format string \"%s\"\n", fmt);
 		exit(EXIT_FAILURE);
 #endif
 	}