Port to anyhow/thiserror

[cgwalters]

The failure crate isn't being actively developed anymore; having
it as a dependency in the public API is a problem for consumers
that don't use it.

Instead, use anyhow/thiserror; in particular the public API for
errors now implements std::error::Error, which allows consuming
applications to e.g. directly use ? rather than needing a
.map_err(|e| e.compat()).

As the docs for thiserror say:

Thiserror deliberately does not appear in your public API. You get
the same thing as if you had written an implementation of
std::error::Error by hand, and switching from handwritten impls
to thiserror or vice versa is not a breaking change.

And we're only using anyhow internally.