Function custom_http_error 

pub fn custom_http_error(
    req: &GatewayRequest,
    err: GatewayError,
) -> GatewayResponse

A custom HTTP error handler that returns JSON error responses.

This handler maps gRPC status codes to HTTP status codes and returns a JSON body with:

• message: The error message.
• status_code: The HTTP status code.
• title: A title for the error (default “Error”).

It falls back to default_error_handler for 2xx codes or codes outside the valid HTTP range [200, 505].