mpg123: improved error report and checks
mpg123: improved error report and checks

Signed-off-by: Carlos Rafael Giani <d7d2e6b3863b2910d2ca34828bc651c85d88df6f@pseudoterminal.org>

GrokImageCompression/gst-plugins-ugly,Kurento/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Kurento/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GStreamer/gst-plugins-ugly,Kurento/gst-plugins-ugly,GStreamer/gst-plugins-ugly