Stephan,

Not sure if it helps, but I generally handled this by send a notification to myself, in plain text. Alternatively, you could collect the error message into a variable and then log the variable somewhere you can read.

Stephen,

Good Morning! I am not sure about the SharePoint online version. The architecture is completely different.

On the On on-premise version, you can look into the SP Logs of the WFE (Web Front End) servers. I believe the workflow message are in 'Workflow Infrastructure' category. Just search by 'Workflow In' in the logs message, you should be able to find relative error messages.

Hope This Helps

Vinay.