ssh_filter_btrbk.sh: return exit status 255 on error

This makes btrbk regard ssh_filter_btrbk errors as ssh errors, as ssh
also returns exit status 255 if an error occurred.

ssh_filter_btrbk.sh

@@ -40,7 +40,7 @@ reject_and_die()
     local reason=$1
     log_cmd "auth.err" "btrbk REJECT" "$reason"
     echo "ERROR: ssh_filter_btrbk.sh: ssh command rejected: $reason: $SSH_ORIGINAL_COMMAND" 1>&2
-    exit 1
+    exit 255
 }
 
 run_cmd()
@@ -154,7 +154,7 @@ while [[ "$#" -ge 1 ]]; do
 
       *)
           echo "ERROR: ssh_filter_btrbk.sh: failed to parse command line option: $key" 1>&2
-          exit 1
+          exit 255
           ;;
     esac
     shift