Enhancement: Return error if no bytes could be read for `backup --stdin`

When restic is used to backup the output of a program, like `mysqldump | restic
backup --stdin`, it now returns an error if no bytes could be read at all. This
catches the failure case when `mysqldump` failed for some reason and did not
output any data to stdout.

https://github.com/restic/restic/pull/2139
