Simulation API runs that complete with 500 errors cause the API to respond with 200 and a `result` of `None`

[anth-volk]

In at least one case logged here, a simulation API run completed with a 500, but the API still responded with a 200 and a result value of None. The intended behavior would be for a 500 from the simulation API to cause a 500 return within the standard API. This does not appear to occur for all simulation API 500s, and my guess is that we're properly handling 500s that occur within the workflow run, but not cases where we don't even initiate the workflow, as happened in the linked case.