gst/playback/gstplaybin.c: Post missing-plugin messages also when we error out because converters, textoverlay or aut...

Original commit message from CVS:
* gst/playback/gstplaybin.c: (post_missing_element_message),
(gen_video_element), (gen_text_element), (gen_audio_element),
(gen_vis_element):
Post missing-plugin messages also when we error out because
converters, textoverlay or auto*sinks are missing (#161922).