src.ctf.fs: append error cause when no traces are found in directory

src.ctf.fs: append error cause when no traces are found in directory

This is just to have at least one way to test the CLI error reporting
with a component actor.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Iaf59163123c48944168e4e6d0ad1c87ea51cdc5a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1619
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>