Recently I've been pulling a lot of grammars into GitHub's highlighting backend,
replacing legacy language support with tree-sitter highlighting queries.
Our backend systems have a standard set of highlight captures we expect, very
similar to the standard tagging captures we expect. Though end-user applications
are free to choose whatever tagging nomenclature they want, I think it's nice to
include a checking stage that will help us ensure that we know whether a capture
might be recognized or not. It will also help us figure out where we need to
expand our standard set of captures (see #1539).
This patch updates the CLI to use anyhow and thiserror for error
management. The main feature that our custom `Error` type was providing
was a _list_ of messages, which would allow us to annotate "lower-level"
errors with more contextual information. This is exactly what's
provided by anyhow's `Context` trait.
(This is setup work for a future PR that will pull the `config` and
`loader` modules out into separate crates; by using `anyhow` we wouldn't
have to deal with a circular dependency between with the new crates.)
