Switch from `error-chain` to `snafu`

`error-chain` seems pretty deprecated, even if it is maintained now.

<https://github.com/rust-lang-nursery/error-chain/issues/254>
<https://users.rust-lang.org/t/error-chain-is-no-longer-maintained/27561>
<https://users.rust-lang.org/t/snafu-0-2-1-released/27269>
<https://docs.rs/snafu>