sink.utils.dummy: append error cause when call to bt_message_iterator_next()` fails

I'm looking at some error stacks for a graph involving a dummy
component, and I noticed that it did not produce an error cause when
failing to consume.  I think it would be nice if it did, to avoid having
a gap in the error stack.

Change-Id: I1b7af3218f9937a21d3dee7406a2b03e122671c6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12007
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>