Provide more meaningful error with unknown client ID

[RubenVerborgh]

When the client ID is unknown, the result is a 401 Unauthorized without any explanation. I propose to make this a 400 instead, with an appropriate error message.

This would help us find bugs such as nodeSolidServer/node-solid-server#592 faster in the future.

[dmitrizagidulin]

Thanks Ruben! :)