Spectrum1D default loader doesn't read in uncertainties for JWST s3d data

When JWST s3d (spectral cube) data is read with Spectrum1D.read, it is not recognized as a known datatype, so the
default loader is used. This loader reads in the flux data, but not the uncertainty data. Loaders should be modified to locate the ERR extension and use that as the Spectrum1D uncertainty.

Start at the Spectrum1D.read default loader path and trace how JWST s3d spectral-cube data is recognized and loaded. Verify that flux is read while the ERR extension is omitted; done means the loader uses ERR as the Spectrum1D uncertainty.

Written by the indexing model from the issue text.