fs: throw errors on invalid paths synchronously

- Throw getPathFromURL() and nullCheck() errors synchronously instead
  of deferring them to the next tick, since we already throw
  validatePath() errors synchronously.
- Merge nullCheck() into validatePath()
- Never throws in `fs.exists()`, instead, invoke the callback with
  false, or emit a warning when the callback is not a function.
  This is to bring it inline with fs.existsSync(), which never throws.
- Updates the comment of rethrow()
- Throw ERR_INVALID_ARG_VALUE for null checks

Author: joyeecheung