Canbus is a method of communication. Using digital signals, instead of analog (ex 12V signal) In canbus systems the modules (individual computers) talk over a network (the bus) to give eachother information. This is no different than 2 computers over the internet.

In analog systems the devices in the system function when a voltage is delivered, rather than bits and bytes (canbus).

LEDs draw less current than your halogen bulbs.that's why your car thinks there is a bulb out.

You can use simple power equations to calculate the resistance you need to trick your car into thinking the bulbs are working.

You'll need input voltage, and the current your setup draws to calculate the resistor you need to get rid of the errors.