Be more specific on HTTP error handling when fetching the batch ID associated to an upload

Error

When resuming an upload, we first check for the batch ID validity using that pseudo-code:

try:
    get_batch_details(BATCHID)
except Exception:
    log.warning("No batch ID found, restarting the upload from zero")

This is bad: we are catching all exceptions. And if the server is busy (likely when uploading a big file) it may return a HTTP error 502, 503 or 504. And those errors are not telling us that the batch ID is inexistant.

Fix

We should only catch the specific HTTP error 404.
Other errors will propagate to the upper level and will be handled accordingly.