Your comment doesn't clear up what you want to achieve, but to me it seems you don't understand how error handling works in Mendix. I suggest you take a look at this learning path.
As additional clarification: the ThrowWebserviceException will always result in an error. This action does not catch an error, but creates one instead. If you have no error handling on the calling microflow, this microflow will not ‘complete with error’, but it is aborted – and changes are rolled back – immediately upon reaching the error. If you debug the flow, you will see that the execution of this microflow stops at the Java action: the red end event will not be reached.