errors : Derive Errors from Exception

According to the docs user defined Exceptions should derive from
Exception not BaseException.

src/unisync/errors.py

@@ -1,11 +1,14 @@
 # Copyright (C) 2025-2026 Paul Retourné
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-class RemoteMountedError(BaseException):
+class RemoteMountedError(Exception):
     pass
 
-class InvalidMountError(BaseException):
+class InvalidMountError(Exception):
     pass
 
-class UnknownSSHError(BaseException):
+class UnknownSSHError(Exception):
+    pass
+
+class FatalSyncError(Exception):
     pass