Better document our error codes

[QuintinWillison]

I'm not entirely sure what a state of completeness looks like on this issue but this does need some thought.

I was looking at this addition to ably-cocoa. While these look like magic numbers, they are also present in errors.json. And their implementation is duplicated in connectionerror.js in ably-js. And, of course, they're variously documented in the features spec.

This clearly relates to #34 which seems to have had some work done on it.

It's clearly beneficial for customers to have more detail on our error codes, but it also feels to me like we need some information for client library developers expanding on what is appropriate use for a particular code. Perhaps that should always be in the features spec but I'm sure I've seen source code where error codes look like they've been 'dropped in'.

[QuintinWillison]

Related, in terms of how we create new error codes for problems that arise client-side: ably/ably-java#757 (comment)