Saves URI to tracker file if one was passed to constructor.

def _save_tracker_uri_to_file(self):
        """
        Saves URI to tracker file if one was passed to constructor.
        """
        if not self.tracker_file_name:
            return
        f = None
        try:
            f = open(self.tracker_file_name, 'w')
            f.write(self.tracker_uri)
        except IOError, e:
            raise ResumableUploadException(
                'Couldn\'t write URI tracker file (%s): %s.\nThis can happen'
                'if you\'re using an incorrectly configured upload tool\n'
                '(e.g., gsutil configured to save tracker files to an '
                'unwritable directory)' %
                (self.tracker_file_name, e.strerror),
                ResumableTransferDisposition.ABORT)
        finally:
            if f:
                f.close()