net: lib: coap: Translate handler errors to CoAP response codes

The CoAP request handler returns errno codes in the following cases:
* ENOENT if no handler found; respond with 4.04
* ENOTSUP if an unknown request code received; respond with 4.00
* EPERM no handler found for the method; respond with 4.05

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>