Raise warning from getx() when it falls back to feature indexing

The fallback in `getx()` often causes user confusion. 

https://github.com/Quasars/orange-spectroscopy/blob/34e964e941077057084046e33756f023d5c0b4b3/orangecontrib/spectroscopy/util.py#L5C1-L15C13

I suggest raising a warning that can be surfaced by the widgets most impacted (Spectra, HyperSpectra, etc)