lib: append error in simple_sink_consume only if error status

If the consume_func returns _AGAIN, we will append an error cause.  That
is wrong because _AGAIN is not an error cause.  Check for negative
status values instead of just != OK.

Change-Id: Ibc03a1e9eb25de5ec39af5148c8e235498c57b64
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2358
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>