Handle expired fileUploadUrl's better

[HowardWolosky]

Some users are attempting to upload their zip files to a submission whose fileUploadUrl has already expired. When they do so, they see this error:

Exception calling "GetResult" with "0" argument(s): "The transfer failed: The remote server returned an error: (403) Forbidden. Server failed to authenticate the request. Make sure the value of Authorization header is formed correctly including the signature.."

We should look into either catching/parsing that error and providing a more helpful one (the fileUploadUrl is typically only valid for 24 hours after the submission has been created) or we should just add a new FAQ to address this issue, in order to help users out.