Is FallbackBroadcasted session event variant necessary?

[spacebear21]

I'm not sure that this new variant is necessary, wouldn't a Failure or Cancel be appropriate?

Originally posted by @spacebear21 in #1061 (comment)

[zealsham]

I think it's a choice of explicity stating that a fallback TX was broadcasted vs implicitly assuming that it was. For the failure/cancel events, depending on the reason of failure/cancel fallback TX can be broadcasted, while adding fallbackbroadcasted event basically tell that the sessionevent transitioned to a state where the fallback tx was definitely broadcasted.