Hi. I believe there is a feature missing from REST API. According to https://docs.camunda.org/javadoc/camunda-bpm-platform/7.9/org/camunda/bpm/engine/delegate/BpmnError.html the BpmnError can have an optional message field. This is also visible in modeler, as we can assign this message to a variable.
Sadly its not possible to set this message when returning bpmn error from external task: https://docs.camunda.org/manual/7.9/reference/rest/external-task/post-bpmn-error/
If its a valid request I would love to have it fixed as soon as possible 