The %ActionResult% token returns text indicating the success or failure of the immediate past action.
Note that this value is reset at the end of every action, so immediately after the Action, you must save the %ActionResult% token into a variable, or use it in a control flow action, like a If/Then Action.
You can use this token for error handling, much like a traditional try/catch/on error construct. But you have to build your own test immediately after the action. Keyboard Maestro will not throw an error in most cases.
Test if the Copy Action failed due to timeout (likely due to no selection)
Action Result, Action Results, Action Failure, Action Success