conn: map sub-error type to UserRequestError

Ultimately, we want to get rid of From<RequestError> for QueryError
implementation. This is because, RequestError contains a
CqlResponseParseError which is overloaded with variants such
as CqlAuthChallengeParseError which should not be returned to the user
who uses only BATCH, QUERY, EXECUTE and PREPARE requests.

This is why, we make two transitions in error types in this place.
The first transition is RequestError -> UserRequestError (map_err()),
which filters out variants such as CqlAuthChallengeParseError, and leaves
only either CqlErrorParseError or CqlResultParseError.
The second transition (? operator) is UserRequestError -> QueryError
which makes use of From<UserRequestError> for QueryError implementation.

scylla/src/transport/connection.rs

@@ -1247,7 +1247,8 @@ impl Connection {
         loop {
             let query_response = self
                 .send_request(&batch_frame, true, batch.config.tracing, None)
-                .await?;
+                .await
+                .map_err(UserRequestError::from)?;
 
             return match query_response.response {
                 Response::Error(err) => match err.error {