text: Title: Test for raised errors in `Scattering2D.forward`
  Body: One `TypeError` and three `RuntimeError`s may be raised by `forward`, but are not currently tested. For example, see [here](https://codecov.io/gh/kymatio/kymatio/src/82f21ed11e9152cd234e63af96d48300210429a9/kymatio/scattering2d/scattering2d.py#L122).