Refactor End-State messages for TaskRunner

[pr1sm]

Describe the bug
Currently the task runner doesn't have a good way to tell if a task has been stopped. This causes the frontend to fall out of sync in its redux store.

To Reproduce
Steps to reproduce the behavior:

1. Create and run a task that errors out
2. Verify that the end status message is posted (Task has error out or something similar)
3. Check the redux state of that task in the frontend and it is probably still running

Expected behavior
When the task ends, the frontend should update its redux state accordingly so the state is kept in sync.

Desktop (please complete the following information):

• OS: MWL
• Service: task-runner
• Version: alpha

Additional context
We can test the incoming message part of the status update to determine if a task has finished, but that's not a stable way to check.

[pr1sm]

This issue can be solved relatively easy by sending a status key along with the message. This will allow the frontend to be notified of a change in state from the TaskRunner and enable it to update its state accordingly.

Specifically, the message here can be updated to include a status key. this would be a short, one-word status update such as errored, finished, or aborted.
https://github.com/walmat/nebula/blob/bfff2b05394d36324ed1cdfdec240a990ba245d8/packages/task-runner/src/shopify/taskRunner.js#L311)