Fallback retry conditions: split up RSC15d into individual items, add a new one

[SimonWoolf]

per https://ably.atlassian.net/browse/INC-230

[owenpearson]

I don't know under which conditions you would get a 400+ error from cloudfront so can't comment on that, but in terms of SDK implementation changes, all LGTM

[QuintinWillison]

Noticing that this pull request is targeting the main (default) branch, the 'bleeding edge' of the current specification version 1.2, rather than the integration/protocol-2.0 branch (#108)... I think we should simplify these statements, as the new form guidance is only introduced from specification version 2.0.0 (#120).

(or, alternatively, if you prefer then you could rebase this PR atop the v2 integration branch)

[SimonWoolf]

Given we're making it on ably-go which does not confirm to 2.0 spec, I think it probably makes sense to keep this pr against main. Made those changes