Don't set pydevd.connected=True, if the connection can't be established.

This change prevents spurious error messages, if application code calls
pydevd.settrace a second time after a failed connection attempt.

```
Traceback (most recent call last):
  File "F:\eclipse\plugins\org.python.pydev_3.1.0.201312121632\pysrc\pydevd_frame.py", line 234, in trace_dispatch
    self.doWaitSuspend(thread, frame, event, arg)
  File "F:\eclipse\plugins\org.python.pydev_3.1.0.201312121632\pysrc\pydevd_frame.py", line 38, in doWaitSuspend
    self._args[0].doWaitSuspend(*args, **kwargs)
  File "F:\eclipse\plugins\org.python.pydev_3.1.0.201312121632\pysrc\pydevd.py", line 1022, in doWaitSuspend
    self.writer.addCommand(cmd)
AttributeError: 'NoneType' object has no attribute 'addCommand'
```