Sanity-check if data is returned for orbital-projected bands

[ajjackson]

In #254 the user sees a confusing error message related to a missing spin key from the orbital-projected data dictionary.

In fact, bs.get_projections_on_elements_and_orbitals has returned an empty dictionary. We could explicitly check for that scenario and raise a more informative error (suggesting to check if LORBIT was set...)