Azure-sdk-for-net: Make ServiceBusException.FailureReason no longer a nested public type

[MiYanni]

Instead, make it into something like ServiceBusFailureReason. No need to have it nested in ServiceBusException.

[jsquire]

For context, the decision was made to nest it in the Event Hubs design because the type is not intended to stand on its own; it has meaning only within the context of that specific exception type. I don't recall there being a strong opinion during architect review, but we may want to be sure that there is buy-in and also consider whether it makes sense to keep the same pattern between the messaging services.

[JoshLove-msft]

Ah yes, we have the same meaning here, but if I recall during the API Review, @tg-msft questioned nesting it.
But if EH is already doing it this way, perhaps it is not worth diverging.

[tg-msft]

Yes, I probably did. This feels a lot like the {Service}ErrorCode pattern we have for all the other libraries and I didn't understand why it's different here. Let's chat about it for 2 minutes at tomorrow's team meeting?

[MiYanni]

As per discussion offline, it seemed like we leaned toward doing this, despite it being against what EH has. The rationale was it is more consistent with a similar pattern in the rest of the SDKs.

[JoshLove-msft]

/cc @KrzysztofCwalina any objections to lifting this out of ServiceBusException so that it is no longer nested?