In the case of PlanetPress Image software(PPImageSoft), the On Error would only be triggered if the error occurs in the PlanetPress Image plugin(PPImagePlug).

PPImageSoft runs asynchronously from Workflow. So the PPImagePlug only gives the job to PPImageSoft.

To trigger an Error Handler upon PPImageSoft, open the PPImageSoft and set the user you which to send an error message.

You will see option to attached the error logs and file in error.

Hope that helps.
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