Add more info to ServicesStartingException

[rpappalax]

{
  "error": "ServicesStartingException",
  "cause": "{\"stackTrace\": [[\"/var/task/handler.py\", 33, \"wait_for_cluster_ready\", \"return runner.wait_for_cluster_ready()\"], [\"/var/task/ardere/step_functions.py\", 299, \"wait_for_cluster_ready\", \"raise ServicesStartingException()\"]], \"errorType\": \"ServicesStartingException\"}"
}

[rpappalax]

per @pjenvey
"EP-default" 48 instances, it's stuck at 47
.... we really need to slightly overprovision the instance count so 1 stuck one doesn't ruin test plan.
Note that the test plan does complete as "successful," but it just sits at the WaitForClusterReady for awhile and then just ends. (no actual load ever hits the server)

[rpappalax]

@pjenvey I suppose there's no practical way to determine more specifically what's going on when the step functions throws this ServicesStartingException? (aka: the only info you're getting back from ECS is that something went wrong)