H.263: more descriptive error message

patch by Tom Joseph < tom-lists @@ @ @@ jambon @@ dot @@ ll @@ dot @@ cx >

Originally committed as revision 4812 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -1139,7 +1139,7 @@ int MPV_encode_init(AVCodecContext *avctx)
         break;
     case CODEC_ID_H263:
         if (h263_get_picture_format(s->width, s->height) == 7) {
-            av_log(avctx, AV_LOG_INFO, "Input picture size isn't suitable for h263 codec! try h263+\n");
+            av_log(avctx, AV_LOG_INFO, "The specified picture size of %dx%d is not valid for the H.263 codec.\nValid sizes are 128x96, 176x144, 352x288, 704x576, and 1408x1152. Try H.263+.\n", s->width, s->height);
             return -1;
         }
         s->out_format = FMT_H263;