When all slaves for a build die, the build becomes immortal

If I create a job that does something terrible and the build brings down all the slaves initially assigned to it, that build will remain in the `BUILDING` state forever with no chance of ever completing. The expected behavior is that the build would enter `ERROR` state with an appropriate failure reason.

This is related to #116 since it's very easy to create a job whose builds will bring down all assigned slaves (all that's needed is a bad command in `setup_build`).