Return error response if assertion fails in SoapUI

I would like to return a custom error message if an assertion fails in SoapUI.

I have written assertions. I always get OK response even though the assertion fails.

I tried below script:

def assertionList = []

def idNotNull = (id!=null) ? "(id is not null" : assertionList.add("(id is null")

if(!assertionList.isEmpty())
{
    return "exceptionResponse"
}
assert assertionList.isEmpty() : assertionList.toString()

But this will return before assert is executed. Hence assertion is passed though it should fail.

Is there a way that I can achieve this?