refactor(keel): use SpinnakerRetrofitErrorHandler with KeelService

[Pranav-b-7]

This PR lays the foundational work for upgrading the retrofit version to 2.x, specifically focusing on refactoring the exception handling for KeelService.

Note, there's a behaviour change on the Task Results error message format when KeelService API throws any 4xx/5xx http errors with empty error body.

• On any 4xx http errors with empty error body:

  before:

  11:56:19.324 [Test worker] ERROR com.netflix.spinnaker.orca.keel.task.ImportDeliveryConfigTask - {message=Non-retryable HTTP response 400 received from downstream service: HTTP 400 http://localhost:62130/delivery-configs/: 400 Bad Request}

  after:

  12:00:02.018 [Test worker] ERROR com.netflix.spinnaker.orca.keel.task.ImportDeliveryConfigTask - {message=Non-retryable HTTP response 400 received from downstream service: HTTP 400 http://localhost:62275/delivery-configs/: Status: 400, URL: http://localhost:62275/delivery-configs/, Message: Bad Request}

• On any 5xx http errors with empty error body:

  before:

  TaskResult(status=RUNNING, context={repoType=stash, projectKey=SPKR, repositorySlug=keeldemo, directory=., manifest=spinnaker.yml, ref=refs/heads/master, attempt=2, maxRetries=5, errorFromLastAttempt=Retryable HTTP response 500 received from downstream service: HTTP 500 http://localhost:65311/delivery-configs/: 500 Server Error}, outputs={})

  after:

  TaskResult(status=RUNNING, context={repoType=stash, projectKey=SPKR, repositorySlug=keeldemo, directory=., manifest=spinnaker.yml, ref=refs/heads/master, attempt=1, maxRetries=5, errorFromLastAttempt=Retryable HTTP response 500 received from downstream service: HTTP 500 http://localhost:49862/delivery-configs/: Status: 500, URL: http://localhost:49862/delivery-configs/, Message: Server Error}, outputs={})

Note : The RetrofitError catch block will still be relevant even after the code changes in the below classes, due to the reasons mentioned inline:

• ImportDeliveryConfigTask.kt - (IgorService retrofit client is not configured with SpinnakerRetrofitErrorHandler , so the APIs still throw RetrofitError)
• DeleteApplicationTask.groovy - (Front50Service retrofit client is not configured with SpinnakerRetrofitErrorHandler, so the APIs still throw RetrofitError)