Allowed exceptions in exercises should accept descendant exceptions

[David Coles]

At the moment we can assign exception names to a TestSuite of an Exercise to say that this exception is allowed for these test cases. Unfortunately, due to the way that we test against exception names we can't handle exception inheritance and polymorphism, so specifying "Exception" as an allowed exception will allow Exception and only Exception exceptions (and none of it's descendants like you would probably expect)

Would require either turning the string back into an exception (possibly a bad idea for secuity reasons) or somehow working out the descendants of an exception from a string.