GSUP: Use SubscriberNotFoundException

[Takuto88]

Note: IMHO it would make sense to define a SubscriberNotFound exception and use it here instead of using the generic ValueError exception as otherwise this can mask other errors and return a misleading error message. I ran into this last week with a similar code path. This doesn't need to block the PR though since this was just moved around.

Originally posted by @osmith42 in https://github.com/nickvsnetworking/pyhss/pull/283#discussion_r2576036537