[one-way-mirror] Uses a dedicated "already running" exit code

By using a dedicated exit code when script detects it is already running I make caller able to easily detect such non-erroneous state.

Author: C-Duv

rsync/one-way-mirror/one-way-mirror-rsync.sh

@@ -122,7 +122,7 @@ fi
 exec 9>$lockFilepath
 if ! flock -n 9 ; then
     echo "I can tell $0 script is already running ($lockFilepath is present since $(stat -c %z $lockFilepath))... exiting this instance"
-    exit 1
+    exit 3
 fi