Make quiz error disappear once checkbox is checked
Make quiz error disappear once checkbox is checked

This one is a little bit tricky because we sync
the "fake but good-looking" checkbox with
a "real but ugly checkbox".

Refs #1270.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2