Improve orchestrator Tester model error handling

[binarymist]

Orchestrator Tester models startTester, initTester and plan:

• Need to retry if requests to Testers fail. Swap Wreck for got as Wreck doesn't support retry
• Provide error handling for failed requests.
  • plan already has some, but it'll probably need changing
  • initTester should be easy to handle as we can just pass back a "Tester failure:"
  • startTester may be a little more work,
    • startTester needs to set isFinished to true and return a "Tester failure:" message, which would also need to be augmented into the combinedInitTesterResponses that the orchestrator returns to the CLI
    • The following could be done, but may not need to be:
    • #warmUpTestSessionMessageChannels should probably also check that it's only acting on Tester models where testerFinished() is true
    • There is also no point in the CLI asking for Tester updates from the orchestrator

May as well swap axios and http-proxy-agent for got and hpagent in app.emissary.js of the app-scanner.

Add tests, even if manual