Prevent writing of file if request was not successful

[jjnesbitt]

Currently, the request to retrieve certain files from DKC often fails with a 502 Bad Gateway, but the file is still written to anyway. Since the file is only written to the first time, this means that if this bad request happens when first running populate, the bad file will remain there until the file item is deleted.

[annehaley]

I agree we should add this line, but I'm not sure it closes the issue. Tests that encounter the nginx 502 error will still fail, albeit with a more informative exception raised.

[jjnesbitt]

I agree we should add this line, but I'm not sure it closes the issue. Tests that encounter the nginx 502 error will still fail, albeit with a more informative exception raised.

You're right, it more just fixes the issue for local dev, since you can just re-run it until it succeeds. I'll remove the link to that issue so this PR won't close it, and reword the issue a bit.