Implement new UI component supporting camunda:errorEventDefinition for external ServiceTasks

[MaxTru]

What should we do?

CamBPM will support camunda:externalEventDefinitions for external service tasks starting with 7.15. (see issue).

Goal of this issue is, to support defining, editing and deleting camunda:externalEventDefinitions for external service tasks.

In particular:

• Add a new section "Errors thrown" to the "Input/Output" tab for serviceTasks with implementation==external.
• Re-use the "list of collapsibles" UI element, we used for input and output parameters
• The "list of collapsbile" headers shall display <errorName> (code: <errorCode>) <- <expression> (note that the expression part shall only be displayed, if an expression is defined and if the item is collapsed.
• Fields as sketched in the UI
• Note that some fields modify the referenced global error entity and not the camunda:errorEventDefinion per se

Why should we do it?

See overall objective camunda/camunda-modeler#2070
Child of camunda/camunda-modeler#2070

[MaxTru]

Update to requirements:
As @andreasgeier pointed out correctly, the "Expression" should also have a validation.

@azeghers please investigate whether "must have value" or "must contain valid expression" validation is better from the user perspective.

[azeghers]

Noted 👍

[azeghers]

Testing session with Miklas and Thorben on 2021/03/10 :

Rough recreation of test case

Tested in cockpit with an hardcoded ${true} throw expression, and with a boolean variable in the external task.
Both resulted in the camunda:ErrorEventDefinition being triggered, and the error path was taken.