Add a nice error message if user tries to decorate a method with `@sematic.func`

[augray]

class Foo:
    @sematic.func
    def bar(self):
        pass

should ideally give an error at time of wrapping rather than runtime, and have a nice message.

[mstew2]

Hello! Can I work on this issue?

[augray]

Sorry for not seeing your comment earlier. Yes, please do!

[tscurtu]

Fixed by #297.