lavc/videotoolboxenc: warn when alpha quality not set

Added a warn log when alpha quality cannot be set, but continue encoding.

Signed-off-by: Rick Kern <kernrj@gmail.com>

libavcodec/videotoolboxenc.c

@@ -1262,6 +1262,13 @@ static int vtenc_create_encoder(AVCodecContext   *avctx,
                                           compat_keys.kVTCompressionPropertyKey_TargetQualityForAlpha,
                                           alpha_quality_num);
             CFRelease(alpha_quality_num);
+
+            if (status) {
+                av_log(avctx,
+                       AV_LOG_ERROR,
+                       "Error setting alpha quality: %d\n",
+                       status);
+            }
         }
     }