errors : Add program ending function

Add a function that quits the program using sys.exit.
This is useful when we enconter a fatal error.

src/unisync/errors.py

@@ -1,6 +1,9 @@
 # Copyright (C) 2025-2026 Paul Retourné
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+from typing import NoReturn
+import sys
+
 class RemoteMountedError(Exception):
     pass
 
@@ -12,3 +15,7 @@ class UnknownSSHError(Exception):
 
 class FatalSyncError(Exception):
     pass
+
+def unisync_exit_fatal(reason:str) -> NoReturn:
+    print(reason)
+    sys.exit(1)