Cryptic error message about "started_at" when there are no simulation results

[nmerket]

Describe the bug
When all the simulations in a buildstockbatch run fail, or when all the jobs run out of time on Eagle, there aren't any simulation_outputs/results_job*.json.gz files. This causes results_df here to be empty or to not have all the columns necessary and to throw and error about there not being the expected started_at column here.

To Reproduce
Steps to reproduce the behavior:

1. Set up a standard buildstockbatch run, but set the minutes per simulation on Eagle to something low enough the jobs won't finish.

-- or --

1. Put a poison pill in every simulation (like missing weather files) that will cause all simulations to fail.

Expected behavior
The current error is confusing and doesn't tell a user what went wrong, only a side effect of what went wrong. We should catch this condition that and raise a more informative error.

Logs

Platform (please complete the following information):

• Simulation platform: Eagle mostly, but could happen on any platform.
• BuildStockBatch version, branch, or sha: v0.17
• OpenStudio-BuildStock version, branch, or sha: any
• Local Desktop OS: not relevant