No connection to Gemini generates unintelligible error messages

[bgottula]

When there is no network connection to Gemini, verbose and generally unhelpful error messages such as below result. Do some basic connectivity testing during construction or find another way to make this less ugly.

(venv) rgottula@nix:~/dev/track$ ./track/align.py --mount-pole-alt=38
Traceback (most recent call last):
  File "/home/rgottula/dev/point/point/gemini_backend.py", line 190, in execute_one_command
    buf_resp = self._sock.recv(self.UDP_RECV_BUF_SIZE)
socket.timeout: timed out

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./track/align.py", line 364, in <module>
    main()
  File "./track/align.py", line 211, in main
    mount = track.LosmandyGeminiMount(args.mount_path)
  File "/home/rgottula/dev/track/track/mounts.py", line 393, in __init__
    accel_limit=max_slew_accel,
  File "/home/rgottula/dev/point/point/gemini.py", line 77, in __init__
    self.set_double_precision()
  File "/home/rgottula/dev/point/point/gemini.py", line 413, in set_double_precision
    self.exec_cmd(G2Cmd_SetDblPrecision())
  File "/home/rgottula/dev/point/point/gemini.py", line 94, in exec_cmd
    return self._backend.execute_one_command(cmd)
  File "/home/rgottula/dev/point/point/gemini_backend.py", line 198, in execute_one_command
    raise G2BackendReadTimeoutError('gave up after {:d} NACK retry attempts'.format(retry_num))
point.gemini_exceptions.G2BackendReadTimeoutError: gave up after 5 NACK retry attempts
Exception ignored in: <bound method Gemini2.__del__ of <point.gemini.Gemini2 object at 0x7fb20c95a710>>
Traceback (most recent call last):
  File "/home/rgottula/dev/point/point/gemini.py", line 91, in __del__
    self.stop_motion()
  File "/home/rgottula/dev/point/point/gemini.py", line 650, in stop_motion
    (actual_rate_ra, limits_exceeded) = self.slew('ra', 0.0)
  File "/home/rgottula/dev/point/point/gemini.py", line 571, in slew
    time_since_last = current_time - self._cached_slew_rate[axis + '_last_cmd_time']
AttributeError: 'Gemini2' object has no attribute '_cached_slew_rate'