In a Workflow you can use Error and/or global error handler available in the right side panel of the Workflow canvas (as you pointed out)
Think of the Error handler action catching the error on one Workflow step. If you have specific step(s) that you want to catch any exceptions that occur, drag and error action from the panel and drop it on the canvas. Then connect the source step to the Error step. This is the similar idea as a Catch in other programming languages.The source step connected to the Error step is like a Try. The error action enables you to do more fine grained error handling at an Workflow step level.
On the other hand, the global error handler will be called if the exception is not caught by an error action in the same Workflow. Make sense?
You can always create a shared error handling workflow and call it from your parent Workflow using the “Run Workflow” action (from the right panel).
If you are talking about wM.io Integration FlowServices, they can use error handling constructs similar to the on-premise wM Integration Server (e.g. TryCatch, If/else, SEQUENCE)
Please clarify if you are talking about wM.io Integration Workflows or FlowServices.
If you need further clarification let me know.
Principal Instructor at Software AG
For: webMethods.io Integration and on-premise integration products.