I don't know GROOVY sorry...
<soap:Fault>
<faultcode>soap:Server</faultcode>
<faultstring>Server was unable to process request. ---> Format of the initialization string does not conform to specification starting at index 0.</faultstring>
<detail/>
</soap:Fault>
So for example the above is an error I get when I do not use the correct connection string properties. I would setup a XPath assertion like this to look for the failure...
XPath Expression:
declare namespace soap='http://schemas.xmlsoap.org/soap/envelope/';
//soap:Fault
XPath Result:
<soap:Fault xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<faultcode>soap:Server</faultcode>
<faultstring>Server was unable to process request. ---> Format of the initialization string does not conform to specification starting at index 0.</faultstring>
<detail/>
</soap:Fault>
Basically the easiest way to do it is to create the TestCase and send a request that fails. Then Add Assertion, and use the Response to create your assertion. Then ever time that test case is run it will check for the failure, and if it doesn't fail in the same way or if it passes it will fail the test.
Most of this can be done automatically by just click Add Assertion, picking XPath Match, then clicking on Declare, and Select from current. You can modify the declare so that you only look for a specific section of the response, then click 'Select from current' and it will import from the current response you have.