Add methods for detecting retriable and fatal Ably exceptions

[KacperKluka]

Added helper methods that will enable us to detect fatal and retriable errors. This should allow us to implement appropriate retry mechanisms for certain Ably operations (like presence enter and leave).

Open questions:

• should we only look at the code property or maybe also at the statusCode?
• how should we classify exceptions that have codes that doesn't start with either 4 or 5?

[paddybyers]

See inline comment

[KacperKluka]

I don't think there's any case that's not a 4xx or a 5xx. I think I'd prefer that there's an error logged in that case - even if statusCode == 0.

@paddybyers I've created an issue for this (#931) as this is a bit tricky to do right and seems a bit out of scope of this PR 😉

Conversation resolved.