Refactoring of errors

[loewenheim]

So far, I’ve only removed the failure dependency.

[bcmyers]

@loewenheim, again, thanks for the PR! This seems to be failing for a similar reason to your other PR. In the areas of the code that are not explicitly marked [cfg(features = "std")], we can't use anything from the standard library; everything has to come from core instead. I'm going to do some thinking about how I'd like errors to work; so perhaps we should table this PR for now.

[bcmyers]

See comment above.