Add error code(s) for rate limiting

We need a new error code to report back to clients that the client was rate limited. As the list of error codes isn't final we should extend the error map with a new "attribute" named "rate-limit" so that clients may be able to know what kind of error the "temp failures" come from.

Given that the attribyte isn't listed in the current spec for the error map we might break clients which don't ignore unknown attributes. If that is the case (that some of our clients perform strict validation of the attributes) we need to update the version number of the error map and clients need to ask for version 2 (and be prepared to receive version 1 if the server don't know about version 2). The server would need to reply with version 1 (without "rate-limit" attribute) to all clients asking for version 1.