Bugfix: Improve error handling for rclone and REST backend over HTTP2

When retrieving data from the rclone / REST backend while also using HTTP2
restic did not detect when no data was returned at all. This could cause
for example the `check` command to report the following error:

    Pack ID does not match, want [...], got e3b0c442

This has been fixed by correctly detecting and retrying the incomplete download.

https://github.com/restic/restic/issues/2742
https://github.com/restic/restic/pull/3453
https://forum.restic.net/t/http2-stream-closed-connection-reset-context-canceled/3743/10
