previously we'd see an error message in the logs:
`AttributeError: 'NoneType' object has no attribute 'status_code'`
because we were assuming the requests exception would always have a
response - it won't have a response if it wasn't able to create a
connection at all.
if doc download returns a 403, that's a screw-up on our side. it's not
helpful to a notify user for that to be passed on. the only thing they
should care about is if it's a 400, because they uploaded a filetype we
don't allow.
Everything else should return 500 internal server error.
Catches the requests exception for document-download-api calls, logs
a warning and returns a matching response code and message.
Connection errors to document download result in 503 response to the
user.
Allows uploading documents to the Document Download API.
The client is configured with an API host and auth token. There's
no need for a flag to disable the client in the test environments
at the moment since the upload is only triggered by a specific
payload which would only be sent with an explicit goal of using
document download.
