when user schema is undefined, should throw better error

[HananoshikaYomaru]

see https://discord.com/channels/1308395750564302952/1420307078479872142.

when user schema is undefined, /api/system/schema?config=1 just return 404.

it will be helpful if user see a better error

[dswbx]

Thanks for reporting! A fix is coming to 0.18 (see commit)