related to #9274, another place needs exception handling

Example traceback:
```
    Traceback (most recent call last):
      File "/usr/lib/pymodules/python2.7/salt/minion.py", line 695, in _thread_return
        return_data = func(*args, **kwargs)
      File "/usr/lib/pymodules/python2.7/salt/modules/ps.py", line 65, in top
        user, system = process.get_cpu_times()
      File "/usr/lib/python2.7/dist-packages/psutil/__init__.py", line 364, in get_cpu_times
        return self._platform_impl.get_cpu_times()
      File "/usr/lib/python2.7/dist-packages/psutil/_pslinux.py", line 327, in wrapper
        raise NoSuchProcess(self.pid, self._process_name)
    NoSuchProcess: process no longer exists (pid=1068)
```