Are customised error messages supported in Custom Tasks?

[fabhdz]

hi experts

I havent been able to find any documentation about this: Are customised error/alert messages supported in custom tasks? (the ones we create in message definition and plug as a "on save" rule, for validation purposes). I have configured one, however instead of showing my customised error message, it only shows "application error". I'm wondering if these are supported to begin with, before i continue to spent any more time on them. 

Thanks