Allow messages to be saved in prepareRow()

It doesn't seem to just be an issue with knowing the key. It looks like messages are cleared at the beginning of the import() step, after all of the prepareRow functions are called, so even if the ID was known in the prepare phase, the messages would be cleared right afterwards.

Perhaps we could move that message clearing to the beginning, as soon as the migration is called? In the meantime, I added some custom functions to my migration class to save messages from the prepareRow functions and add them during the import phase:

  /**
   * saveMessage doesn't work in prepare function because import clears them, so save them for later
   */
  public function savePrepareMessage($message, $level = MigrationBase::MESSAGE_NOTICE) {
    $prepareMessage->message = $message;
    $prepareMessage->level = $level;
    $prepareMessage->sourceKey = $this->source->getCurrentKey();
    $this->prepareMessages[] = $prepareMessage;

    // trigger an error so it shows up when drush runs the migration
    if(function_exists("drush_log")) drush_log($message.": ".$prepareMessage->sourceKey['path']);
  }

  /**
   * save the messages after the normal import function is complete
   */
  protected function import() {
    $return = parent::import();

    // get the messages saved in prepareMessage
    if(isset($this->prepareMessages)) {
      foreach($this->prepareMessages as $message) {
        $this->map->saveMessage($message->sourceKey, $message->message, $message->level);
      }
    }
    return $return;
  }