FIX: Pass API errors to frontend during job execution failures

[jakereps]

Expands the try block on the job execution endpoint to include the async call, allowing for the passing of the matplotlib backend errors.

Fixes #68

@ebolyen, @Kleptobismol can you review?

[thermokarst]

Confirmed that the mpl backend error case is caught by this, although I wonder if using a different status code makes more sense (like a 500, because the error is originating from the server). At the same time, it probably doesn't matter.

[ebolyen]

I agree that 500 makes sense for this, but we'd have to differentiate between error types (possible, but sounds like a pain at the moment). Probably the best way to handle this would be to have a specific set of Exception subclasses which we can recognize, and then anything else becomes a 500. We could probably write a decorator of some kind so that every request can handle exceptions uniformly (then it's just a matter of consistently doing something useful with it on the front-end).

[ebolyen]

This looks good to me, thanks @jakereps!

[jakereps]

Thanks for testing, @thermokarst! I think a few of the status codes on the backend don't exactly fit what they are doing, but our front end doesn't really differentiate using the codes (yet?). Our fetch call used for all endpoint communications just uses if (!response.ok) { ...throw Error... }, which I think only looks if it's in the 200s.