Feature/575 error notifications validation

Partially fixes #575. 

I added validation. Also now we display the right value in the dropdown and in the chatter group textarea (if chatter group selected) when editing the page, except the first time. Unfortunately,  if chatter group was the stored value, when we edit for the first time the dropdown and the chatter group textarea don't get populated. I think they might be due to the combination of using rerender and JQuery. However, this is taking me too much time to try get working, and I believe this acceptable for the time being.