Robust electron

Closes #17 (closed).

In this order:

• Check that parity exists on disk
• If it does not exists, then download it from vanity
  • Retry 3 times if there's an error, show error message after those 3 retries
  • After successful download, download checksum and verify it
• Check if there's a process (using ps-node) that's named parity
  • If yes, then don't run parity
  • If no, then run parity with --light flag
• Use more professional logging (with pino instead of console.log, see screenshot). Outputs in stderr and in log file.

If it's working well I'll port it to the shell too.