Crash if no audio system found
I know that I'm in a weird environment where my pc actually have an audio card (on the mainboard) but I don't have the drivers (and don't want them as it is a work PC and audio is not something I need here)

So when Pyxel try to find and get the audio card it "crash":

```
python3 pyxel_examples/01_hello_pyxel.py 
Traceback (most recent call last):
  File "pyxel_examples/01_hello_pyxel.py", line 20, in <module>
    App()
  File "pyxel_examples/01_hello_pyxel.py", line 6, in __init__
    pyxel.init(160, 120, caption='Hello Pyxel')
  File "/usr/local/lib/python3.6/dist-packages/pyxel/__init__.py", line 33, in init
    border_width, border_color)
  File "/usr/local/lib/python3.6/dist-packages/pyxel/app.py", line 87, in __init__
    self._audio_player = AudioPlayer()
  File "/usr/local/lib/python3.6/dist-packages/pyxel/audioplayer.py", line 143, in __init__
    callback=self._output_stream_callback)
  File "/usr/local/lib/python3.6/dist-packages/sounddevice.py", line 1373, in __init__
    **_remove_self(locals()))
  File "/usr/local/lib/python3.6/dist-packages/sounddevice.py", line 696, in __init__
    extra_settings, samplerate)
  File "/usr/local/lib/python3.6/dist-packages/sounddevice.py", line 2489, in _get_stream_parameters
    info = query_devices(device)
  File "/usr/local/lib/python3.6/dist-packages/sounddevice.py", line 487, in query_devices
    raise PortAudioError('Error querying device {0}'.format(device))
sounddevice.PortAudioError: Error querying device -1
```

I think it should gracefully  says that audio is not going to work because it can't find a valid sound card, but should not fail because of that.
