ctf: msg-iter.c: use `_APPEND_CAUSE` variants of logging macros

ctf: msg-iter.c: use `_APPEND_CAUSE` variants of logging macros

This commit also changes usage of logging level macros `LOGW` in this
file to use `LOGE` which is more appropriate when returning
_STATUS_ERROR. All callers of this API consider _STATUS_ERROR as
un-recoverable.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ie44d98e0285bdcf2c3ed7f2aeeb1af0cfadef5d4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2243
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>