Use actix_web::Error for top level error handling.

[trueleo]

On handlers for endpoints, instead of using HttpResponse as return type it should be replaced with Result<HttpResponce, actix_web::Error>. This change is tied up to #2 and can't be implemented without these being worked on first.

Reference

• https://actix.rs/docs/errors/
• https://www.lpalmieri.com/posts/error-handling-rust/

Todo

• [ ] Have smaller error types with appropriate error messages.
• [ ] Conversion from these custom error types to actix_web::Error