Add validator, to run in CI, that ensures `errors.json` only contains codes below `100000`

[QuintinWillison]

This is because we've decided that codes from 100000 are to be canonically defined outside the scope of this repository. They are reserved for use by specific Ably projects and ecosystems.

[QuintinWillison]

Follows on from #162.

[mattheworiordan]

FYI. We now use error codes >= 100000, see https://github.com/ably/ably-common/tree/main/protocol.