Provide informative error when "invalid_account_type/feature/" happens on files/upload

[rianhunter]

Right now when a "invalid_account_type/feature/" error happens on files/upload the Python SDK throws an error of UploadError('other', None) instead of something more informative. Referenced here: https://github.com/rianhunter/dbxfs/issues/17

[greg-db]

Thanks for opening this Rian. I'll follow up here when we have an update on it.