bug 881561 - correlations service should not error for missing report

@rhelmer r?

So, with this, if we get a 404 from crash-analysis on the GET, this means that mware will now return Null (which is a valid JSON payload).

If some other error happens, the `DownloadError` will bubble up and there's nothing we can do really in socorro-crashstats to wrap that other than to let us know.