Handle failing commands more gracefully

[patricoferris]

This PR adds changes OBuilder to keep failed builds in the ZFS and Rsync backends -- for now they are kept and not distinguished from other builds.

On top of this Shark will now stop processing a markdown file once it encounters a failing command. This includes a single failing command inside a map. I would like to change quite substantially how Shark processes maps and builds in general to support this better but for now this seemed sensible as a first PR. The failed command output is promoted back to the code block and the rest of the markdown file is ignored. On exit we print the failing command and the markdown and return a non-zero exit code.

The CI runs a slightly incorrect markdown file where we expect the gdal --version command to fail as there is no gdal command.