dblink_error_message fetches the most recent remote
    error message for a given connection.
   
    When asynchronous queries are initiated by
    dblink_send_query, the error message associated with
    the connection might not get updated until the server's response message
    is consumed. This typically means that dblink_is_busy
    or dblink_get_result should be called prior to
    dblink_error_message, so that any error generated by
    the asynchronous query will be visible.