Downloading a non-series'ified patch with dependencies causes a HTTP 500 error

[stephenfin]

As noted in getpatchwork/git-pw#14, we're not properly handling patches without series attached to them.

[stephenfin]

I'm not sure what to do in this case. I imagine the correct thing is probably a HTTP 404 since we can't correctly satisfy the request. However, we could also just ignore the parameter. Hmm...