Improve error reporting for invoking non-existent method from FnTesting

[zootalures]

Given a class under test, when FnTesting.thenRun(cls, method) is run where method doesn't exist on cls, the test will fail with

java.lang.IllegalStateException: One and only one response expected, but 0 responses were generated..

Whilst true, this is misleading, we should catch this earlier in the test framework and let the user know that the desired method doesn't exist.