partial downloaded model is not handled correctly .

[ftao]

The problem is partial download is treated as successful download after restart stt-model-manager . A simple solution can be :

1. using a tmp file name when downloading , and rename it after successful download.
2. check file size and continue download after restart

How to reproduce

1. open stt-model-manager
2. click "Install a new model from the Coqui STT Model Zoo."
3. choose any model to install , eg [English STT v0.9.3], wait the first file
4. stop stt-model-manager before finish downloading ( at process 10%)
5. restart stt-model-manager , reinstall the model , the process jump to 50% , wait to 51%
6. stop stt-model-manager and restart again , the model is in "Installed STT models" section.
7. try to run the model will cause Segmentation fault: 11