MDEV-14150 - backup should fail early if rsync is missing

check availability of rsync utility early , when starting backup with
--rsync. Fail if it is not there.

extra/mariabackup/xtrabackup.cc

@@ -6165,9 +6165,19 @@ xb_init()
 		return(false);
 	}
 
-	if (opt_rsync && xtrabackup_stream_fmt) {
-		msg("Error: --rsync doesn't work with --stream\n");
-		return(false);
+	if (xtrabackup_backup && opt_rsync)
+	{
+		if (xtrabackup_stream_fmt)
+		{
+			msg("Error: --rsync doesn't work with --stream\n");
+			return(false);
+		}
+		bool have_rsync = IF_WIN(false, (system("rsync --version > /dev/null 2>&1") == 0));
+		if (!have_rsync)
+		{
+			msg("Error: rsync executable not found, cannot run backup with --rsync\n");
+			return false;
+		}
 	}
 
 	n_mixed_options = 0;