Change Error enum to be more semantically meaningful

[heartsucker]

I keep tacking stuff on to it, but realistically it should have just an Opaque error type to catch all the IO / HTTP / whatever stuff, and then other values for TUF failures.

[heartsucker]

So apparently there's this black magic stuff called error-chain that I should probably be using because omg every time I have to debug a nested error, I want to die.

[tarcieri]

error-chain is pretty nice for managing boilerplate and automatically generating backtraces