better error processing for rnm-us

[kflemin]

• when simulation results look like this:
  {
  messages: "",
  results: {
  message: "could not find results file Summary.json"
  },
  simulation_id: "c6d1cec4-fb3b-4393-bcfe-a9f0e8837eee",
  status: "completed"
  }

1. allow for downloading of the errors.txt file (even if that's the only file that got created)
2. ensure that all output files were generated before returning a status of "completed" (which kind of implies success).
3. in rnm-us gem: check that there are results in the response and not just this type of message with no real results.