specification: Add means to throw an error

What would you like to be added?

Add an action that allows throwing an error

Why is this needed?

Allows to define and throw workflow-specific errors, which is not possible as of now.

Coupled with the action’s condition property, it could provide users a very powerfull - yet extremely simple - tool to perform flow and/or data validation logic.

What is your proposal?

Create a new throw or error action type, that allows users to do the following:

actions:
  - name: is-room-available
    functionRef:
      refName: check-if-room-is-available
      arguments:
        id: 123
    actionDataFilter:
      toStateData: .roomAvailable
  - name: throw-if-not-true
    condition: ${ .roomAvailable == false }
    errorRef:
      refName: room-not-available

If, as suggested in #770, we get rid of the top-level errors property, then it could instead look like:

actions:
  - name: is-room-available
    functionRef:
      refName: check-if-room-is-available
      arguments:
        id: 123
    actionDataFilter:
      toStateData: .roomAvailable
  - name: throw-if-not-true
    condition: ${ .roomAvailable == false }
    throw:
      type: https://fake.hotel.com/errors/room-not-available
      status: 409
      title: Room Not Available