Descriptive error for post processing empty results

[aspeake]

Fixes #151

Pull Request Description

Throws a more descriptive error in post-processing when no simulation results are found

Checklist

Not all may apply

• Code changes (must work)
• Tests exercising your feature/bug fix (check coverage report on CircleCI build -> Artifacts)
• All other unit tests passing
• Update validation for project config yaml file changes
• Update documentation
• Run a small batch run to make sure it all works (local is fine, unless an Eagle specific feature)

[aspeake]

@nmerket I have added a error message when no results are found in postprocessing.py, and have checked that this error works locally, but have not written any tests for it or run on Eagle yet.

Also, I am not sure if we need a more descriptive message, I figured this issue could arise a few different ways (eg, timeout or all sims failed), so it is currently pretty vague.

[nmerket]

@aspeake This is a good start. I might just raise an exception with the error message. It's probably fine the way it is as far as descriptiveness. Just knowing that much will give people a clue where to start looking next.

[nmerket]

This looks good to me. One suggestion below. I think it's ready. If you do too, mark it ready for review so I can merge it.

[aspeake]

@nmerket this should be ready for review + merge