Make XLogRecGetBlockTag() throw error if there's no such block.

All but a few existing callers assume without checking that this
function succeeds.  While it probably will, that's a poor excuse for
not checking.  Let's make it return void and instead throw an error
if it doesn't find the block reference.  Callers that actually need
to handle the no-such-block case must now use the underlying function
XLogRecGetBlockTagExtended.

In addition to being a bit less error-prone, this should also serve
to suppress some Coverity complaints about XLogRecGetBlockRefInfo.

While at it, clean up some inconsistency about use of the
XLogRecHasBlockRef macro: make XLogRecGetBlockTagExtended use
that instead of open-coding the same condition, and avoid calling
XLogRecHasBlockRef twice in relevant code paths.  (That is,
calling XLogRecHasBlockRef followed by XLogRecGetBlockTag is now
deprecated: use XLogRecGetBlockTagExtended instead.)

Patch HEAD only; this doesn't seem to have enough value to consider
a back-branch API break.

Discussion: https://postgr.es/m/425039.1649701221@sss.pgh.pa.us

src/backend/access/transam/xlogutils.c

@@ -369,7 +369,8 @@ XLogReadBufferForRedoExtended(XLogReaderState *record,
 									&prefetch_buffer))
 	{
 		/* Caller specified a bogus block_id */
-		elog(PANIC, "failed to locate backup block with ID %d", block_id);
+		elog(PANIC, "failed to locate backup block with ID %d in WAL record",
+			 block_id);
 	}
 
 	/*