Fix: cli: use BT_CLI_LOGE_APPEND_CAUSE instead of printf to print errors

While fixing up the test cli/convert/test_convert_args, I noticed that
some error messages were printed on stdout, and not using error causes.
Indeed, they are printed directly using printf.

Change them to use BT_CLI_LOGE_APPEND_CAUSE.

This is tested by a following patch which updates
cli/convert/test_convert_args.

Change-Id: I18a901d50b643293dd806c1fbe7d2372dc8bd46f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3146
Tested-by: jenkins <jenkins@lttng.org>