Users with read only access not given any feedback on failed uploads

[zeigerpuppy]

I made a mistake today and shared a davros grain read only to a colleague. The user was trying to upload files and the upload would appear to progress and then just not complete. It would be good to have a warning at the start for that user that informs them to request write access to the sandstorm grain (and doesn't waste their bandwidth with an upload that is destined to fail on write!).