fixed ffmpeg error reporting on Python 3

I got the following error:

    /usr/local/lib/python3.5/dist-packages/moviepy/audio/io/ffmpeg_audiowriter.py:91: in write_frames
        + "\n\n"+ffmpeg_error))
    E   TypeError: Can't convert 'bytes' object to str implicitly

To fix it, I wrapped `ffmpeg_error` in `str()` in `ffmpeg_audiowriter.py`.  I also did the same in `ffmpeg_writer.py` but I didn't test the latter.

moviepy/audio/io/ffmpeg_audiowriter.py

@@ -88,7 +88,7 @@ def write_frames(self,frames_array):
             ffmpeg_error = self.proc.stderr.read()
             error = (str(err)+ ("\n\nMoviePy error: FFMPEG encountered "
                      "the following error while writing file %s:"%self.filename
-                     + "\n\n"+ffmpeg_error))
+                     + "\n\n" + str(ffmpeg_error)))
 
             if b"Unknown encoder" in ffmpeg_error:
 

moviepy/video/io/ffmpeg_writer.py

@@ -138,7 +138,7 @@ def write_frame(self, img_array):
             _, ffmpeg_error = self.proc.communicate()
             error = (str(err) + ("\n\nMoviePy error: FFMPEG encountered "
                                  "the following error while writing file %s:"
-                                 "\n\n %s" % (self.filename, ffmpeg_error)))
+                                 "\n\n %s" % (self.filename, str(ffmpeg_error))))
 
             if b"Unknown encoder" in ffmpeg_error: