Don't fail import when files (pictures/avatars) are missing from local path [patch]

I used various examples I found in the issue queues to come up with the following for importing users with pictures. Due to the issues with the http-based file copy, I manually copied the avatars to the pictures directory and have copy_file set to false. My problem is that the avatars are named (avatar_{UID}.png) based on the UID, which always exists, but not every user has an avatar. The import is working for users that have avatars, but not for those who don't with error: filesize() [function.filesize]: stat failed for ...

Is it possible to detect the missing avatar and default the picture value to 0 for those users that don't have one so that they may still be imported?

Here's my code:

class UsersMigration extends TUMigration {
  public function __construct() {
    parent::__construct();

    $this->description = t('Migrate TU Users');

    $this->dependencies = array('Avatars');

    $source_fields = array(
      'id_member' => t('User ID'),
      'roles' => t('The set of roles assigned to a user.'),
    );

    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'id_member' => array(
          'type' => 'int',
          'unsigned' => TRUE,
          'not null' => TRUE,
          'description' => 'SMF Unique User ID',
          'alias' => 'u',
        )
      ),
      MigrateDestinationUser::getKeySchema()
    );

    $query = Database::getConnection('default', 'tuprod')
      ->select('base_members', 'u')
      ->fields('u', array('ID_MEMBER', 'memberName', 'dateRegistered', 'passwd', 'emailAddress'))
      ->condition('u.is_activated', 1, '=')
      ->condition('u.ID_MEMBER', 1, '>');

    $this->source = new MigrateSourceSQL($query, $source_fields, NULL, array('map_joinable' => FALSE));

    $this->destination = new MigrateDestinationUser();

    $this->addFieldMapping('uid', 'id_member');
    $this->addFieldMapping('name', 'membername');
    $this->addFieldMapping('pass')->defaultValue('');
    $this->addFieldMapping('mail', 'emailaddress');
    $this->addFieldMapping('theme')->defaultValue('');
    $this->addFieldMapping('signature')->defaultValue('');
    $this->addFieldMapping('created', 'dateregistered');
    $this->addFieldMapping('status')->defaultValue(1);
    $this->addFieldMapping('language')->defaultValue('');
    $this->addFieldMapping('init', 'emailaddress');
    $this->addFieldMapping('picture', 'user_avatar')->sourceMigration('Avatars');
    $this->addFieldMapping('is_new')->defaultValue(TRUE);

  }
}

class AvatarsMigration extends Migration {
  private $base_path;
  public function __construct() {
    parent::__construct();

    $this->base_path = 'public://' . variable_get('user_picture_path', 'pictures') . '/';

    $this->description = t('User avatars images');

    $source_fields = array(
      'id_member' => t('User ID'),
      'roles' => t('The set of roles assigned to a user.'),
    );

    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'id_member' => array(
          'type' => 'int',
          'unsigned' => TRUE,
          'not null' => TRUE,
          'description' => 'SMF Unique User ID',
          'alias' => 'u',
        )
      ),
      MigrateDestinationFile::getKeySchema()
    );

    $query = Database::getConnection('default', 'tuprod')
      ->select('base_members', 'u')
      ->fields('u', array('ID_MEMBER'))
      ->condition('u.is_activated', 1, '=')
      ->condition('u.ID_MEMBER', 1, '>');

    $query->distinct();

    $this->source = new MigrateSourceSQL($query, array(), NULL, array('map_joinable' => FALSE));

    $this->destination = new MigrateDestinationFile(array(
      'copy_file' => false,
      'preserve_files' => true,
    ));

    $this->addFieldMapping('uri', 'id_member')->callbacks(array($this, 'add_path'));
    $this->addUnmigratedDestinations(array('fid', 'uid', 'status', 'filemime', 'timestamp'));

  }

  public function add_path($uri) {
    return $this->base_path . 'avatar_' . $uri . '.png';
  }
}