sink.text.pretty: simplify error handling in pretty_consume

sink.text.pretty: simplify error handling in pretty_consume

Replace the switch with a simple condition.  On _END, it is not
necessary to put the message iterator, that will be done in
pretty_finalize.

Change-Id: Ia0bd20e0d119cae155c03cc5d7656a5c4619ebb8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3406
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>