Validate against trying to override non-overrideable method

[rcodesmith]

If you try to override a method that is not overrideable (e.g. because the return type wasn't declared and is inferred) and call the next object in the override dispatch chain, Sculptor doesn't give a useful error.

What happens is when the 'next' method is called, it ends up calling the base implementation of the method in the dispatch object, and it'll frequently fail with an NPE because the dispatch object hasn't had any objects injected into it. I got an error like the following, along with a stack trace pointing to the base class method implementation where it tries to use an injected object:

[ERROR] Error running action 'org.sculptor.generator.template.RootTmpl.root': null

We should add a validation that guards against attempts to override methods that are not overrideable in Sculptor (e.g. because the return type is inferred).