Return Value on UPDATE trigger

On a table update, I want to check for certain conditions in a trigger
function, and if a condition is met, NOT ALLOW the update.  Not a big
problem...I can accomplish that.

My question is this:  How can I send a message back to the calling
application telling the user that the update was denied or failed, and for
the user to try again?  Basically, I need a way to send a return value back
to the application (a 0 or a 1) when a trigger function says so.

Any ideas?

Thanks,
Derrick