Style fix and docstring for the APM QA Python tool

webrtc/modules/audio_processing/test/py_quality_assessment/quality_assessment/exceptions.py

 # Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 """Exception classes.
 """
 
 
 class FileNotFoundError(Exception):
+  """File not found exeception.
+  """
   pass
 
 
 class SignalProcessingException(Exception):
+  """Signal processing exeception.
+  """
   pass