Right-click on the 'call webservice' action. Choose ErrorHandling, select custom with/without rollback. Create a new action above the action. Connect it to the webservice action, right click on the connection, 'set as error handler'.
Do your custom error handling there, anything you can do in Mendix (Log, save a Log record, show a custom message, retry a second endpoint).
I will reuse this thread because the title still applies perfectly to my question:
Modeler will raise an error on integration (as in Mugilan's case) or database errors and the user has an option to handle and re-throw the error.
But what if I want to raise an error on purpose, for the calling microflow to handle. Such as validating all my input parameters and if any are null, throw an error (design by contract pattern). Modeler doesn't let me to designate the 'false' branch of an exclusive split, for example, to be the "set as error trigger", or something like that...