lib: plugin-so.c: ERROR and INFO level messages logged in callee

lib: plugin-so.c: ERROR and INFO level messages logged in callee

Failing to load a plugin from a file is not an error as we often need to
try to load shared libraries to tell if they actually contain a
Babeltrace plugin.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ic5b72e0f0c050b7e11896d264787d852139eaf56
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1701
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>