public void noExceptionThrown() {
    Throwable thrown = getSpecificationContext().getThrownException();
    if (thrown == null) return;
    throw new UnallowedExceptionThrownError(null, thrown);
  }
Specifies that no exception should be thrown, failing with a
{@link UnallowedExceptionThrownError} otherwise.
spock-core/src/main/java/spock/lang/Specification.java