examples/demuxing_decoding: print the decoding error when it happens.

doc/examples/demuxing_decoding.c

@@ -76,7 +76,7 @@ static int decode_packet(int *got_frame, int cached)
         /* decode video frame */
         ret = avcodec_decode_video2(video_dec_ctx, frame, got_frame, &pkt);
         if (ret < 0) {
-            fprintf(stderr, "Error decoding video frame\n");
+            fprintf(stderr, "Error decoding video frame (%s)\n", av_err2str(ret));
             return ret;
         }
 
@@ -99,7 +99,7 @@ static int decode_packet(int *got_frame, int cached)
         /* decode audio frame */
         ret = avcodec_decode_audio4(audio_dec_ctx, frame, got_frame, &pkt);
         if (ret < 0) {
-            fprintf(stderr, "Error decoding audio frame\n");
+            fprintf(stderr, "Error decoding audio frame (%s)\n", av_err2str(ret));
             return ret;
         }
         /* Some audio decoders decode only part of the packet, and have to be