Helpful error message if someone puts @Inject on a class, not NPE

If someone is silly enough to put @Inject on a class, they might expect an error telling them not to. However, the annotation processor runs on this dodgy code and the InjectProcessor gets given this to deal with. It was throwing a NullPointerException which took some time to track down, so I've put in a simple little check for it and made it report a nice helpful error message instead. I've added a test for this too.