pg_upgrade: revert part of patch for ease of translation

Revert part of 959ee6d2 .

Backpatch-through: 10

src/bin/pg_upgrade/server.c

@@ -310,8 +310,13 @@ start_postmaster(ClusterInfo *cluster, bool throw_error)
 	 * running.
 	 */
 	if (!pg_ctl_return)
-		pg_fatal("pg_ctl failed to start the %s server, or connection failed\n",
-			cluster == &old_cluster ? "source" : "target");
+	{
+		/* keep error strings separate to ease translation */
+		if (cluster == &old_cluster)
+			pg_fatal("pg_ctl failed to start the source server, or connection failed\n");
+		else
+			pg_fatal("pg_ctl failed to start the target server, or connection failed\n");
+	}
 
 	return true;
 }