storage: Adjust to changes in privileged errors

Tests:

- Edit auto-mounting to try mounting an already mounted device. In a VM, during
service startup, notice the warning about failure to auto-mount as device is
already mounted.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/storage/udisks2.py

@@ -196,7 +196,7 @@ def _mount(object_path):
     try:
         privileged.mount(block_device.preferred_device, _log_error=False)
     except Exception as exception:
-        stderr = exception.args[3].decode()
+        stderr = getattr(exception, 'stderr', b'').decode()
         if 'GDBus.Error' not in stderr:
             raise