Error code documentation is out-of-date

[alecgibson]

The error codes documented at the end of the README are out-of-date. For example, according to the README, error 4002 means "Database adapter does not support subscribe". However, according to the source code, error 4002 means "Op submit rejected".

I'm happy to raise a pull request to delete these from the README to at least reduce confusion, but probably don't have the time to rummage through the code to update the docs.

[nateps]

Thanks for flagging this!

Fixing the error codes is something I'd like to do for a 1.0 release, and this should be a relatively quick update.

As of Node.js 8.0, Node uses the convention of Error.code being a string, which in retrospect makes more sense than a number. I'd like to update both the codes in ShareDB and the README to be a unique string in a similar convention to the one used by Node.js: https://nodejs.org/api/errors.html#nodejs-error-codes

I was planning to make this change, but if you'd like to suggest some codes, I'd appreciate the help!