Relax attribute lookup in warning about old-styled middleware

For class-based middleware like
```
class Middleware:
    async def __call__(self, app, handler):
          ...
```
there is no `__name__` attr.
Moreover I could imaging other situations where `__name__` doesn't exists.
It breaks web server starting.

Let's change `middleware.__name__` to `repr(middleware)`.

Bugfix should be done in `2.3` branch.