API should return error on unsupported fields

[shyuep]

mpr.materials._search(nelements=2, fields=["material_id", "final_structure"]) simply returns the material_id and the final_structure is ignored. This is generally not a good behavior for people to learn that final_structure is not actually a supported field for materials.

[munrojm]

I agree with this, we should check all fields requested against MPRester.*.available_fields and notify people if anything they ask for is invalid.