Add more meaningful error message when accessing specprod database

[weaverba137]

The notebook database/How_to_use_SPARCL_at_NERSC.ipynb makes an implicit assumption that database/spectroscopic-production-database.ipynb has already been run by the user, or that the user has otherwise already set up the necessary database authentication credentials.

Please add additional text, which could be copied directly from the section 'Initial Database Connection' in database/spectroscopic-production-database.ipynb as well as a try/except with a meaningful error message, as in the one code cell in that section.