Better guidance for a failed start instance

[jarekdanielak]

Is your feature request related to a problem? Please describe.

If task testing fails to start an instance, no detailed error is shown, even though it's available on the API.

Describe the solution you'd like

A more detailed error is shown.

[Buckwich]

Here's the (broken) diagram I used:

broken-eventsubprocess-task-testing.bpmn.txt