py: Handle errors when JS calls Python, handle more IO errors

[extremeheat]

• py: errors when Python calls JS which calls Python back which returns an error is now handled
• py: do not block program exit if JS process is dead
• IO errors are suppressed. These errors are common when the other side of the bridge exists before the IO is finished on child process (e.g. crashing or Ctrl-C), no need to log them
• py: add new mineflayer example