Replace more error types by Fallibe.

[adamreichold]

Fallible might not look like a clear win ATM due to the additional .into() calls, but I think this will change when error propagation is used more systematically. For example,

match fs::read_to_string(file_name) {
    Ok(config) => match toml::from_str(&config) {
        Ok(config) => config,
        Err(error) => {
            return Err(format!(
                "ERROR: Could not parse config file \"{}\": {}",
                file_name, error
            )
            .into());
        }
    },
    Err(error) => {
        return Err(format!(
            "ERROR: Could not read config file \"{}\": {}",
            file_name, error
        )
        .into());
    }
}

could be done as

fn read_config(path: &Path) -> Fallible<AnyConfig> {
  let buf = fs::read_to_string(path)?;
  let val = toml::from_str(&buf)?;
  Ok(val)
}
...
let config = read_config(path).map_err(|err| format!("ERROR: Could not read config file \"{}\": {}", path.display(), err))?

which depends on usign Box<dyn Error> to unify std::io::Error and toml::de::Error, but I did not want to mix to many changes into one PR. (I am also not I understand what the occurrences_of == 0 check above that snippet actually does.)

[mlange-42]

I am also not I understand what the occurrences_of == 0 check above that snippet actually does.

@adamreichold You are right, that case should be already covered by the None branch of the enclosing match.