Messages shouldn't contain line breaks

      drupal_set_message(t('You have chosen to subscribe to %list. An email 
        will be sent to your address. Click the link in the email to confirm 
        the subscription.', array('%list' => $list->label)));

This is bad style. Also if you don't know that there is a line break you can't match this string with a regex, as something like '^You have chosen to.*$' will fail.

This would resolve this issue

      drupal_set_message(t('You have chosen to subscribe to %list. An email '
        . 'will be sent to your address. Click the link in the email to confirm '
        . 'the subscription.', array('%list' => $list->label)));