release-notes: Add release notes template for 0.21

This can be filled in as we go along to save having to write the release
notes in their entirety at release time.
stbt batch: Return failure exit status if any of the tests fails

`stbt batch run` now exits with non-zero exit status if any of the tests
in the run failed or errored.  This, in combination with the `-1` option
makes it easier to use from and integrate with external CI systems.