Refactor IxaError to use thiserror

[RobertJacobsonCDC]

Our current error type is IxaError:

pub enum IxaError {
    IoError(io::Error),
    JsonError(serde_json::Error),
    CsvError(csv::Error),
    Utf8Error(std::string::FromUtf8Error),
    ParseIntError(std::num::ParseIntError),
    IxaError(String),
}

But we haven't really taken advantage of it and tend to just use IxaError::IxaError("some error message".to_string()).

Alternatives:

• https://docs.rs/thiserror/latest/thiserror/
• https://docs.rs/anyhow/latest/anyhow/
• Make IxaError richer and use it in more places.

What's easy for client code to integrate?

[k88hudson-cfa]

• Let's use thiserror for all user-facing (and where applicable, internal) errors, replacing IxaError
• Try to create a specific enum variant instead of plain string IxaError
• Avoid using Result<T, String> or other errors that don't implement std::Error