error: define AVERROR_EXTERNAL.

This code is intended for errors in external libraries
when no corresponding error code can be found.
AVERROR_UNKNOWN is too vague for that and looks like AVERROR_BUG.

libavutil/error.c

@@ -37,6 +37,7 @@ struct error_entry error_entries[] = {
     { ERROR_TAG(ENCODER_NOT_FOUND),  "Encoder not found"                              },
     { ERROR_TAG(EOF),                "End of file"                                    },
     { ERROR_TAG(EXIT),               "Immediate exit requested"                       },
+    { ERROR_TAG(EXTERNAL),           "Generic error in an external library"           },
     { ERROR_TAG(FILTER_NOT_FOUND),   "Filter not found"                               },
     { ERROR_TAG(INVALIDDATA),        "Invalid data found when processing input"       },
     { ERROR_TAG(MUXER_NOT_FOUND),    "Muxer not found"                                },

libavutil/error.h

@@ -54,6 +54,7 @@
 #define AVERROR_ENCODER_NOT_FOUND  FFERRTAG(0xF8,'E','N','C') ///< Encoder not found
 #define AVERROR_EOF                FFERRTAG( 'E','O','F',' ') ///< End of file
 #define AVERROR_EXIT               FFERRTAG( 'E','X','I','T') ///< Immediate exit was requested; the called function should not be restarted
+#define AVERROR_EXTERNAL           FFERRTAG( 'E','X','T',' ') ///< Generic error in an external library
 #define AVERROR_FILTER_NOT_FOUND   FFERRTAG(0xF8,'F','I','L') ///< Filter not found
 #define AVERROR_INVALIDDATA        FFERRTAG( 'I','N','D','A') ///< Invalid data found when processing input
 #define AVERROR_MUXER_NOT_FOUND    FFERRTAG(0xF8,'M','U','X') ///< Muxer not found

libavutil/version.h

@@ -31,7 +31,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR 51
-#define LIBAVUTIL_VERSION_MINOR 63
+#define LIBAVUTIL_VERSION_MINOR 64
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \