cli: return if bt_plugin_find_all_from_dir fails in load_dynamic_plugins

cli: return if bt_plugin_find_all_from_dir fails in load_dynamic_plugins

If the call to bt_plugin_find_all_from_dir in load_dynamic_plugins
returns an error (a negative status code), we currently continue
execution as if nothing happened.

This patch makes it so that we exit the loop and report the error to the
caller.

Change-Id: Ib22813b92fe06156cde1681c55651a294cd67909
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1934
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>