Handle errors on the API side

* The map endpoints return an HTTP 404 error in case of unknown or
  out-of-bound locations
* The forecast endpoint with an address returns an HTTP 404 with error
  JSON in case geocoding fails
* The forecast endpoints return the errors per metric in the `errors`
  field of the forecast
* Implement `Display` for `Metric`
* Use a `BTreeMap` to have an ordered `errors` field/object
* Also log the errors to the console
* Update the tests
* Document the errors that can occur

src/lib.rs

@@ -68,6 +68,7 @@ impl<'r, 'o: 'r> rocket::response::Responder<'r, 'o> for Error {
             Error::NoPositionFound => Status::NotFound,
             Error::Maps(MapsError::NoMapsYet) => Status::ServiceUnavailable,
             Error::Maps(MapsError::OutOfBoundCoords(_, _)) => Status::NotFound,
+            Error::Maps(MapsError::OutOfBoundOffset(_)) => Status::NotFound,
             _ => Status::InternalServerError,
         };