Any ideas for how to handle errors?

When uploading or downloading, it's possible that we will encounter an error. How could/should we handle this?

Ideas:

* Set a flag/message in our state and expect the user to check for errors if they care.
* Dispatch a special action which we expect the user to capture/handle.

Start by reading the existing upload and download handling and how state and actions flow through the middleware. Define an error-handling approach for both operations, including how users observe or handle failures; done means the behavior and integration point are agreed and implemented.

Written by the indexing model from the issue text.