JSON factory: a misspelt "type" lists no candidates and a missing "type" leaks the nlohmann exception

[petlenz]

• "type": "j2_plasticty" fails without listing the registered types or the nearest match.
• An entry with no "type" key surfaces the raw nlohmann key 'type' not found exception, with no indication of which material entry it was.

Fix

Catch both in json_material_factory; report the material name (or its index in the deck when the name is missing too), and for an unknown type list the registered ones.

All references are to main at 91bd26f.