Allow using "fail" with a reason that will be shown in the WF execution state info

Currently using "fail" will fail the workflow and the user will need to go over the different entities and flow and find why.
This BP will add the ability to fail the workflow and provide a message that will appear in the workflow execution state info.

For example:

---
version: '2.0'

create_vm_and_send_email:
  type: direct

  input:
    - vm_name
    - image_id
    - flavor_id

  tasks:
    create_vm:
      action: nova.servers_create name=<% $.vm_name %> image=<% $.image_id %> flavor=<% $.flavor_id %>
      publish:
        vm_id: <% task(create_vm).result.id %>
      on-error:
        - send_error_email

    send_error_email:
      action: send_email to_addrs=['<email address hidden>'] body='Failed to create a VM'
      on-complete:
        - fail('Failed to create a VM with image <% $.image_id %>')