FIX ffmpeg: don’t log error on getting stderr

Author: christianbrodbeck

surfer/utils.py

@@ -714,7 +714,8 @@ def ffmpeg(dst, frame_path, framerate=24, codec='mpeg4', bitrate='1M'):
     logger.info(std_info)
     if stderr.strip():
         err_info = os.linesep.join(("FFmpeg stderr", '=' * 27, stderr))
-        logger.error(err_info)
+        # FFmpeg prints to stderr in the absence of an error
+        logger.info(err_info)
 
     # check that movie file is created
     if not os.path.exists(dst):