gstvideometa: do not emit criticals when adding timecode metas

gstvideometa: do not emit criticals when adding timecode metas

This is inconsistent with other add_meta methods such as
gst_buffer_add_video_meta , which will return NULL without
logging when gst_video_info_set_format fails.

It is up to the caller to check the return value of the
function, and log if appropriate.