Show descriptive error when trying to assign a device token to a gateway stake

[PodTheCoder]

Issue encountered by a user on Discord: https://discord.com/channels/371989135172567051/921502088687673375/1199062009422037093

Scenario:
User has created a Gateway stake and tries to assign a Device Token to it. The user enters a correct password.

Response:
forbidden

Expected response:
You cannot (currently) assign a device token to a Gateway.

Scenario might also apply to a Stargate stake.