Raise an exception when nonexistent shared examples are referenced

The following spec will silently pass:

    SpecBegin(_SharedExamplesTest6)

    itShouldBehaveLike(@"nonexistent behavior", nil);

    SpecEnd

I think it should trigger an exception that points out that the shared example group "nonexistent behavior" is undefined. 

This pull request makes it so. However, I couldn't find a way to test the changes since Specta eats up all exceptions that occur outside an example (and then calls `exit(1)`).