mpc8: return more meaningful error codes.

libavformat/mpc8.c

@@ -202,7 +202,7 @@ static int mpc8_read_header(AVFormatContext *s)
     c->header_pos = avio_tell(pb);
     if(avio_rl32(pb) != TAG_MPCK){
         av_log(s, AV_LOG_ERROR, "Not a Musepack8 file\n");
-        return -1;
+        return AVERROR_INVALIDDATA;
     }
 
     while(!pb->eof_reached){
@@ -214,14 +214,14 @@ static int mpc8_read_header(AVFormatContext *s)
     }
     if(tag != TAG_STREAMHDR){
         av_log(s, AV_LOG_ERROR, "Stream header not found\n");
-        return -1;
+        return AVERROR_INVALIDDATA;
     }
     pos = avio_tell(pb);
     avio_skip(pb, 4); //CRC
     c->ver = avio_r8(pb);
     if(c->ver != 8){
         av_log(s, AV_LOG_ERROR, "Unknown stream version %d\n", c->ver);
-        return -1;
+        return AVERROR_PATCHWELCOME;
     }
     c->samples = ffio_read_varlen(pb);
     ffio_read_varlen(pb); //silence samples at the beginning