matroskadec: use more appropriate error code

Originally committed as revision 22535 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index b4b6d5c..1e646fc 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1144,7 +1144,7 @@
                "EBML header using unsupported features\n"
                "(EBML version %"PRIu64", doctype %s, doc version %"PRIu64")\n",
                ebml.version, ebml.doctype, ebml.doctype_version);
-        return AVERROR_NOFMT;
+        return AVERROR_PATCHWELCOME;
     }
     ebml_free(ebml_syntax, &ebml);