Process.cwd() may raise NoSuchProcess instead of ZombieProcess

[giampaolo]

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ERROR: psutil.tests.test_process.TestProcess.test_zombie_process
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Traceback (most recent call last):
  File "psutil/tests/test_process.py", line 1455, in test_zombie_process
    zproc.as_dict()
  File "psutil/__init__.py", line 563, in as_dict
    ret = meth()
  File "psutil/__init__.py", line 739, in cwd
    return self._proc.cwd()
  File "psutil/_pslinux.py", line 1320, in wrapper
    raise NoSuchProcess(self.pid, self._name)
NoSuchProcess: psutil.NoSuchProcess process no longer exists (pid=14147)