Error out on send failure in walsender loop.

I changed the loop in 9.3 to use "goto send_failure" instead of "break" on
errors, but I missed this one case. It was a relatively harmless bug: if
the flush fails once it will most likely fail again as soon as we try to
flush the output again. But it's a bug nevertheless.

Report and fix by Andres Freund.

src/backend/replication/walsender.c

@@ -1291,7 +1291,7 @@ WalSndLoop(void)
 					ping_sent = true;
 					/* Try to flush pending output to the client */
 					if (pq_flush_if_writable() != 0)
-						break;
+						goto send_failure;
 				}
 			}