Make downloading parity more robust
Right now we just try to fetch parity once: https://github.com/parity-js/light/blob/20679c4515cf18570ab751c19c28c2e295854d0a/electron/operations/fetchParity.js#L53-L61
It'd be nice to:
- Retry 2-3 times if the 1st time fails (and show a Check Network Connection message if failing after 3 times)
- Check that the checksum is correct after downloading (or re-download if not)