Improve unsupported (pixel) format error message.

patch by Stefano Sabatini, stefano.sabatini-lala poste it

Originally committed as revision 10996 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/snow.c

@@ -4115,7 +4115,7 @@ static int encode_init(AVCodecContext *avctx)
         s->colorspace= 1;
         break;*/
     default:
-        av_log(avctx, AV_LOG_ERROR, "format not supported\n");
+        av_log(avctx, AV_LOG_ERROR, "pixel format not supported\n");
         return -1;
     }
 //    avcodec_get_chroma_sub_sample(avctx->pix_fmt, &s->chroma_h_shift, &s->chroma_v_shift);