Improve error message in case of output file with no streams, dumping

the empty file format representation and specifying the index number
of the output file.

Originally committed as revision 15870 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1582,7 +1582,8 @@ static int av_encode(AVFormatContext **output_files,
     for(i=0;i<nb_output_files;i++) {
         os = output_files[i];
         if (!os->nb_streams) {
-            fprintf(stderr, "Output file does not contain any stream\n");
+            dump_format(output_files[i], i, output_files[i]->filename, 1);
+            fprintf(stderr, "Output file #%d does not contain any stream\n", i);
             av_exit(1);
         }
         nb_ostreams += os->nb_streams;