plugin/cli: return task result when waiting

When waiting for the task, return the result from the `watch_task`
call, which will indicate if the task was successful. This will
then be reflected in the error code of the koji call.

plugins/cli/osbuild.py

@@ -103,3 +103,4 @@ def handle_osbuild_image(options, session, argv):
     if res == 0:
         result = session.getTaskResult(task_id)
         print(result)
+    return res