Crash if default voice isn't installed.

[kmcnaught]

The default settings specifies voice_info="Microsoft Anna" which isn't installed on my UK system. This causes an exception the first time you try to "Listen", and can be worked around by going to Settings (which populates with available voices) and clicking "Save".

I'll make a PR with a fix. Let me know if you're open to merging a PR or if I should fork the project?