Originally it was thought the upload URL expiring would provide 401
errors so it was excluded from reauth when doing uploads, but on
re-reading the docs and looking at this issue it seems that 401 errors
are only caused by the account token expiring and not the upload token
expiring.
We will refresh both the upload token and account token on a 401 error
while uploading, and just the account token when we get a 401 at any
other time.
