Tolerate 404 responses due to missing folders
This prevents our own mailbox's download worker from getting stuck trying to list a folder that has been removed, or download files from a folder that has been removed. Instead, the worker will move on to the next folder/file.
Note: This should be merged on the same day as briar-mailbox!111 (merged) to avoid breaking the nightly integration tests.
Edited by akwizgran