Bugfix: Return error when no bytes could be read from stdin

We assume that users reading backup data from stdin want to know when no data
could be read, so now restic returns an error when `backup --stdin` is called
but no bytes could be read. Usually, this means that an earlier command in a
pipe has failed. The documentation was amended and now recommends setting the
`pipefail` option (`set -o pipefail`).

https://github.com/restic/restic/pull/2135
https://github.com/restic/restic/pull/2139