Use `snafu` crate for structured error-handling

[julianhyde]

In #21, @ouillie suggested using snafu or thiserror. I think I prefer snafu because it provides an opinionated style (which is good, because I don't know what I'm doing).

[ouillie]

FYI I haven't really used either. I'm more familiar with anyhow because that suits my project better. anyhow is better for standalone applications where you never really catch errors rather than just surfacing them to the user; the human-readable content is most important, and it focuses on attaching extra context to errors as they bubble up the call stack, rather than catching them.

On the other hand, snafu and thiserror are both geared more toward libraries that are consumed by other programs, so modeling your errors as proper enums is more important so they can be efficiently and unambiguously handled programmatically. I noticed you were already using hand-rolled error enums, which is why I suggested those.