Investigate making rust error types use the failure crate

As our Rust code increases, we'll eventually want a nicer way to convert between error types than we currently have. We'll probably want to use boats's failure crate. They mentioned a while ago that they were going to make a 1.0.0 release soon, and afaict there's not really anything about the current release that is expected to change, so we can probably start working on this now-ish.

[update] This ticket is pending stability of the failure crate and the direction of the error trait in Rust https://github.com/rust-lang/rfcs/pull/2504