core: switch to anyhow

This commit adds `anyhow` as a dependency and switches over to it from
Box<dyn Error>.

It actually looks like I've kept all of my errors rather shallow, such
that we don't get a huge benefit from anyhow at present. But now that
anyhow is in use, I expect to use its "context" feature more going
forward.

Cargo.toml

@@ -49,6 +49,7 @@ members = [
 ]
 
 [dependencies]
+anyhow = "1.0.75"
 bstr = "1.6.0"
 grep = { version = "0.2.12", path = "crates/grep" }
 ignore = { version = "0.4.19", path = "crates/ignore" }