Change invalid device instruction response

[kedvall]

Currently, shifu's response for trying to use an instruction that doesn't exist is 404 page not found.

Please change this to something like Error: Device instruction does not exist as it's currently not too clear it's shifu giving back the 404, especially when using a GO driver with an HTTP server.
It would also be convenient if this were configurable, but mainly having a different default would solve it.