omx: don't log error when failing to load conf file with generic target

The generic target is meant to only test building gst-omx. It doesn't
provide any configuration file and so is not supposed to register any
element.
I'm not aware of any user building gst-omx with this target and
providing their own conf file to actually register elements. But best to
not break this use case anyway so let's just downgrade the log message.

Fix GST_ERROR in the 'check fedora' CI job.
9 jobs for generic in 13 minutes and 2 seconds (queued for 1 second)