Improved error handling, partially closes #46

[VictorKoenders]

I saw #46 was closed 5 min ago but I was already working on it, so if you still want it, here it is!

[VictorKoenders]

Failure is my preferred crate for handling errors, for what it's worth.

[17cupsofcoffee]

I seem to have a knack for picking up issues about half an hour before you submit a PR 😅 Sorry about that!

I added a couple of these From implementations in my branch, but missed out the SDL2 ones, so I'll still take this PR :) Can just rebase my changes on top of it.

I quite like Failure too, but it seems like its future is in flux a bit since the RFC for fixing the Error trait went through. Probably going to stick with standard Rust error handling for now, at least until things are a bit more clear on that front.