Add a log message for this job.
string $message: The message to store in the log. Keep $message translatable by not concatenating dynamic values into it! Variables in the message should be added by using placeholder strings alongside the variables argument to declare the value of the placeholders. See t() for documentation on how $message and $variables interact.
string[] $variables: (Optional) An array of variables to replace in the message on display.
string $type: (Optional) The type of the message. Can be one of 'status', 'error', 'warning' or 'debug'. Messages of the type 'debug' will not get printed to the screen.
public function addMessage($message, $variables = array(), $type = 'status');