Implement custom error type

[Shadow53]

• Allows attaching an error message to an HTTP status code
• Implements std::error::Error
• Implements std::fmt::Display
  • Returns the actual error message
• Implements warp::reject::Reject
• Separate method for determining the error message to send back to client
  • If status >= 500, use a generic message rather than the actual error text
  • If status >= 400 && status < 500 use the actual error message

Note: May use an external library for this. I am considering implementing this as part of Nebula as its own crate that we can use here.