Tweak pg_promote() to report failures on kill() or postmaster failures

Since its introduction in 10074651e335, pg_promote() has been returning
a false status in three cases:
- SIGUSR1 not sent to the postmaster process.
- Postmaster death during standby promotion.
- Standby not promoted within the specified wait time.

An application calling this function will have a hard time understanding
what a false state returned actually means.

Per discussion, this switches the two first states to fail rather than
return a "false" status, making the second case more consistent with the
existing CHECK_FOR_INTERRUPTS in the wait loop.  False is only returned
when the promotion is not completed within the specified time (60s by
default).

Author: Ashutosh Sharma
Reviewed-by: Fujii Masao, Laurenz Albe, Michael Paquier
Discussion: https://postgr.es/m/CAE9k0P=QTrwptL0t4J0fuBRDDjgsT-0PVKd-ikd96i1hyL7Bcg@mail.gmail.com

src/backend/access/transam/xlogfuncs.c

@@ -711,10 +711,10 @@ pg_promote(PG_FUNCTION_ARGS)
 	/* signal the postmaster */
 	if (kill(PostmasterPid, SIGUSR1) != 0)
 	{
-		ereport(WARNING,
-				(errmsg("failed to send signal to postmaster: %m")));
 		(void) unlink(PROMOTE_SIGNAL_FILE);
-		PG_RETURN_BOOL(false);
+		ereport(ERROR,
+				(errcode(ERRCODE_SYSTEM_ERROR),
+				 errmsg("failed to send signal to postmaster: %m")));
 	}
 
 	/* return immediately if waiting was not requested */
@@ -744,7 +744,10 @@ pg_promote(PG_FUNCTION_ARGS)
 		 * necessity for manual cleanup of all postmaster children.
 		 */
 		if (rc & WL_POSTMASTER_DEATH)
-			PG_RETURN_BOOL(false);
+			ereport(FATAL,
+					(errcode(ERRCODE_ADMIN_SHUTDOWN),
+					 errmsg("terminating connection due to unexpected postmaster exit"),
+					 errcontext("while waiting on promotion")));
 	}
 
 	ereport(WARNING,