better looking error message when trying to add empty lists

This is an enhancement to #27:
- use bootstrap alert alert-error instead of js alert()
- the alert can be displayed multiple times: dismiss button doesn't remove it from the page, just hide it

TODO: can be enhanced further by implementing the "hide" also for "alert alert-success" (the binding should call isErrorMessageVisible or isInfoMessageVisible depending on the type of the element whose close button was clicked)