fs: remove watcher state errors for fs.watch

- Remove ERR_FS_WATCHER_ALREADY_STARTED and
  ERR_FS_WATCHER_NOT_STARTED because those two situations should
  result in noop instead of errors for consistency with the
  documented behavior of fs.watchFile.
  This partially reverts https://github.com/nodejs/node/pull/19089
- Update comments about this behavior.

Refs: https://github.com/nodejs/node/pull/19089

PR-URL: https://github.com/nodejs/node/pull/19345


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>