Throw error with correct code when node exists

[neezer]

I've the following helper method to create directories:

const mkdirSync = fs => (dirPath) => {
  try {
    fs.mkdirSync(dirPath);
  } catch (e) {
    if (e.code !== 'EEXIST') {
      throw e;
    }
  }
};

Unfortunately, this doesn't work with memfs, as addDir/addFile will throw a non-system error if a node exists (with no code defined).

This PR throws an error closer to what Node itself would actually throw: https://nodejs.org/api/errors.html#errors_system_errors

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Lemme know if you need me to change anything about this PR to get it merged in. Really enjoyed working with memfs, particularly for testing. Thanks for your hard work!

[neezer]

Aph, just realized your using TypeScript as your source... I'll re-open later once I update. Sorry!