Better error handling

[f-forcher]

• Reduce or eliminate unnecessary unwrap/expect use in the code
• Change fallible functions' signatures to Result<...>

Longer term:

• Switch from anyhow crate to custom errors, eg with thiserror crate. This enables better control over the error types and nehaviour, such as the possibility to statically disable expensive backtraces for errors that do not need it, and to attach and filter context to errors.