The errors and warnings in pipeline nodes are handled from the client side. This error check provides you an early and instant feedback on the node errors in a pipeline. This avoids request (REST API call) to the server (and potentially to Hive) and reduces the load on server for errors.
Following are the errors and warnings handled in the User Interface:
A request is sent from the client to the server only for warnings. For errors (or errors and warnings), no request is sent from the client to the server.