Better disk failure detection in the installer

[jdreed]

Several times now I've found myself having to do site visits for failed HDs, because the installer failure mode is odd. Specifically, it doesn't fail during disk detection, but does during partition creation. We already have a partman hook to sanity-check the size of disks, we should consider some sort of timed operation (e.g. write a bunch of bytes to /dev/sda), and then display a "This disk has failed, try again." error or something.