Emit a visible warning when a /dev node is not what we expect

[dankamongmen]

See #125 for an amusing investigation. The problem ended up being that a regular file (containing a valid filesystem, natch) existed at /dev/sdo1. This both confused the hell out of me, and prevented the device node from being properly created by the kernel. When we take a look at a /dev node, if it isn't a S_IFBLK (or maybe a S_IFCHR), we should emit a diagnostic, and ideally somehow highlight this interloper in our UI.