sink.text.pretty: abort on unknown field class type

I don't think there a legimitate case for the print_field function to
encounter an invalid field class type.  Change it to abort instead of
returning an error.

Change-Id: I333d2221b828841aff38a7d8b0a17032af3aafb1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7580
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7573