muxer: append causes on some _next iterator method code paths

The goal of this patch (on top of the fact that having more error causes
is nice) is to ensure that if an error status is returned to
muxer_msg_iter_do_next, an error is set on the current thread.  This
will help for the next patch, which will save and restore the error in
this function.

Change-Id: I7221af107f330ab5ded10b69f24121be6ad1678c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2382
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>