Convert code base to Error/ErrorKind pattern

[ncalexan]

#730 did heroic work to convert from error-chain to failure, but our initial consumer (a logins API that consumes Mentat and is in turn consumed by applications like Mozilla Lockbox) is finding that returning failure::Error isn't very helpful. It looks like we'll need to adopt the Error/ErrorKind pattern from https://github.com/withoutboats/failure/blob/868273409c826755812dbff1b67cc0ac3fa702f7/book/src/error-errorkind.md.

[ncalexan]

@grigoryk I think we talked about this, but clearly I didn't understand the full context of what's involved. I see TODO items in the code to do this work in some places; do you have a strong opinion about this approach?