bake: Better error handling for site baking. The site baker now keeps track of whether any worker saw any error. It returns the current record after a bake. The `bake` command uses this return value to figure out what kind of exit code to return.

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build node_modules venv tmp* {args}