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 xxxxxxxx, got e3b0c442
```

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