Running SoapUI specific methods from an external script

I have an external groovy script that evaluates a response ( "yes" or "no" ). If the response is "no" I want the testRunner to fail. This is the code I have in my code library

package library.serviceAvailabilityResult

import org.apache.commons.lang.RandomStringUtils

class ServiceAvailabilityResult {

 String getServiceAvailabilityResult(String result) {

 String serviceAvailabilityResult

 if (result == "yes") {
 serviceAvailabilityResult = result
 } else if (result == "no") {
 return testRunner.fail('SERVICE IS NOT AVAILABLE')
 }
 }
}

This obviously fails. Does anyone know how to return the 

testRunner.fail('SERVICE IS NOT AVAILABLE')

in a way that SoapUI sees it as a command? 

I am calling the method above in a groovy script that looks like this

def myServiceAvailabilityResult = new ServiceAvailabilityResult()
String result = "no"
String myResult = myServiceAvailabilityResult.getServiceAvailabilityResult(result)
log.info myResult