support for synchronous upload

[wkalt]

It would be nice if the library supported a "synchronous" argument. This could work by using the request ID returned from the import endpoint and polling the /pending-imports endpoint until either a success or failure appears.

Currently the behavior of the backend is to display a failure on retryable failures (which could resolve to a success eventually if it succeeds before the retry count). We probably need to coordinate around that somehow, probably by indicating in the pending-imports response when a failure status is terminal.