docs: manual: improve advanced metadata example a bit
docs: manual: improve advanced metadata example a bit

Accept both filename and a URI as argument, and print
the error from the error message if there's an error.

https://bugzilla.gnome.org/show_bug.cgi?id=756630

centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer