PostProcessingError holds the message in the 'msg' property, not in 'message' (fixes #1323)

Causes DeprecationWarning: http://www.python.org/dev/peps/pep-0352/

youtube_dl/PostProcessor.py

@@ -137,7 +137,7 @@ class FFmpegExtractAudioPP(FFmpegPostProcessor):
         try:
             FFmpegPostProcessor.run_ffmpeg(self, path, out_path, opts)
         except FFmpegPostProcessorError as err:
-            raise AudioConversionError(err.message)
+            raise AudioConversionError(err.msg)
 
     def run(self, information):
         path = information['filepath']
@@ -207,7 +207,7 @@ class FFmpegExtractAudioPP(FFmpegPostProcessor):
         except:
             etype,e,tb = sys.exc_info()
             if isinstance(e, AudioConversionError):
-                msg = u'audio conversion failed: ' + e.message
+                msg = u'audio conversion failed: ' + e.msg
             else:
                 msg = u'error running ' + (self._exes['avconv'] and 'avconv' or 'ffmpeg')
             raise PostProcessingError(msg)