ffmpeg_opt: Give the user a hint on how to ignore unsupported streams

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_opt.c

@@ -2022,12 +2022,16 @@ loop_end:
                     av_log(NULL, ignore_unknown_streams ? AV_LOG_FATAL : AV_LOG_WARNING,
                            "Cannot map stream #%d:%d - unsupported type.\n",
                            map->file_index, map->stream_index);
-                    if (!ignore_unknown_streams)
+                    if (!ignore_unknown_streams) {
+                        av_log(NULL, AV_LOG_FATAL,
+                               "If you want unsupported types ignored instead"
+                               "of failing, please use the -ignore_unknown option\n");
                         exit_program(1);
                     }
                 }
             }
         }
+    }
 
     /* handle attached files */
     for (i = 0; i < o->nb_attachments; i++) {