gstmediafile: improve error message when there is an input stream.

Previously it was unclear why passing an input stream would crash a video,
unless you went into the source and read it.

This adds a clarifying message that only file based media files are supported.

modules/media/gtkgstmediafile.c

@@ -287,6 +287,7 @@ gtk_gst_media_file_open (GtkMediaFile *media_file)
   else
     {
       /* It's an input stream */
+      g_error ("Input Streams are currently not supported. Please pass a File based MediaFile.");
       g_assert_not_reached ();
     }