Better error checking in the backup script.

* scripts/backup.in: Check exit code of MT_BEGIN
* scripts/backup.sh.in (backup_host): Return meaningful code
in case of remote execution.  See the comment, though.

scripts/backup.sh.in

@@ -311,7 +311,9 @@ backup_host() {
         CMD="exec ${TAR_PART1} -f \"${TAPE_FILE}\" $@"
         message 10 "CMD: $CMD"
         sh -c "$CMD"
-        message 10 "RC: $?"
+        RC=$?
+        message 10 "RC: $RC"
+        return $RC
     fi
 }