Swarming bots error out on bad devices.

build/android/pylib/local/device/local_device_environment.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import datetime
 import functools
 import logging
 import os
 import shutil
 import tempfile
@@ skipping 186 matching lines @@
       except base_error.BaseError:
         logging.exception('Failed to stop logcat monitor for %s',
                           m.adb.GetDeviceSerial())
       except IOError:
         logging.exception('Failed to locate logcat for device %s',
                           m.adb.GetDeviceSerial())
 
     if self._logcat_output_file:
       file_utils.MergeFiles(
           self._logcat_output_file,
-          [m.output_file for m in self._logcat_monitors])
+          [m.output_file for m in self._logcat_monitors
+           if os.path.exists(m.output_file)])
       shutil.rmtree(self._logcat_output_dir)
 
   def BlacklistDevice(self, device, reason='local_device_failure'):
     device_serial = device.adb.GetDeviceSerial()
     if self._blacklist:
       self._blacklist.Extend([device_serial], reason=reason)
     with self._devices_lock:
       self._devices = [d for d in self._devices if str(d) != device_serial]