Fix error status of vacuumdb when multiple jobs are used

When running a batch of VACUUM or ANALYZE commands on a given database,
there were cases where it is possible to have vacuumdb not report an
error where it actually should, leading to incorrect status results.

Author: Julien Rouhaud
Reviewed-by: Amit Kapila, Michael Paquier
Discussion: https://postgr.es/m/CAOBaU_ZuTwz7CtqLYJ1Ouuh272bTQPLN8b1bAPk0bCBm4PDMTQ@mail.gmail.com
Backpatch-through: 9.5

src/bin/scripts/vacuumdb.c

@@ -712,7 +712,10 @@ vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 		for (j = 0; j < concurrentCons; j++)
 		{
 			if (!GetQueryResult((slots + j)->connection, progname))
+			{
+				failed = true;
 				goto finish;
+			}
 		}
 	}