Decide correct behaviour when ably-cocoa emits a state change without an error when there should be one

[lawrence-forooghian]

My understanding is that an ARTChannelStateChange emitted by an ably-cocoa channel will have a non-nil reason if one of the following is true (this at least seems to be the assumption made by the chat spec):

• it is an UPDATE with resumed == false
• it is an ATTACHED with resumed == false
• it is a FAILED
• it is a SUSPENDED

However, ably-cocoa’s Objective-C API is incapable of encoding this information, meaning that currently we instead have to trap at runtime (i.e. preconditionFailure) if the above expectation is violated. Decide what's the right thing to do here.

This is a similar issue to #12 (which describes a similar issue that users of the Chat SDK will encounter).

┆Issue is synchronized with this Jira Task by Unito