It's not possible in xwiki for the moment the core send message to the
user for saying something like :
"You attachment have been attached" or "You succesfully logged in"
or replace exception with a readable message like:
"your file is too big to be attached"
what do you think about adding in the context 2 collections:
* one message which contains messages like the first one.
* the other one, which contains errors, like the second kind of message.
After, we display this in the skin in a box under the top menu.
I know message is already on the code, but it's never displayed, and i
think we have to do the distinction between message and error.