Avoid dependencies on (external) error types

[Anyhow](https://docs.rs/anyhow/latest/anyhow/index.html) provides an idiomatic way for passing around errors without the need to keep every single one of them in a combined error enum, which should reduce inter-dependencies without limiting our ability to inspect errors later on.

Start by locating the combined error enums and the places where external error types create dependencies. Review the Anyhow documentation linked in the issue and assess which errors still need structured inspection. Done means the dependency direction is improved without losing the ability to inspect errors later.

Written by the indexing model from the issue text.