undo r22995: we must return an error code when the operation failed

git-svn-id: https://xpra.org/svn/Xpra/trunk@22996 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/server/mixins/server_base_controlcommands.py

@@ -449,8 +449,7 @@ def control_command_clipboard_direction(self, direction, *_args):
         assert self.clipboard and ch
         direction = direction.lower()
         DIRECTIONS = ("to-server", "to-client", "both", "disabled")
-        if direction not in DIRECTIONS:
-            return "supported directions: %s" % csv(DIRECTIONS)
+        assert direction in DIRECTIONS, "invalid direction '%s', must be one of %s" % (direction, csv(DIRECTIONS))
         self.clipboard_direction = direction
         can_send = direction in ("to-server", "both")
         can_receive = direction in ("to-client", "both")