can: update documentation wording error frames -> error messages

As Heinz-Juergen Oertel pointed out 'CAN error frames' are a already defined
term for the CAN protocol violation indication on the wire.

To avoid confusion with the error messages created by CAN drivers available
via CAN RAW sockets update the documentation and change the naming from
'error frames' to 'error messages' or 'error message frames'.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>