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

PR-URL: #18308
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: joyeecheung