How can I signal an error from a velocity template?

I want to write a template that generates different outputs based on a
property value. say the value can be "A" or "B". Now, if the user passes
something else, I want to signal an error. so my template looks something
like:
# if...
.....
#elseif
....
#else
// throw an exception? how?
#end

Thank you,
Ittay