Make the initializers of NIOThreadPoolError public

[omochi]

I tried to implement the following methods in my project to make the NIOThreadPool more user-friendly in a Swift Concurrency environment.

extension NIOThreadPool {
    public func runIfActive<T>(_ body: @escaping @Sendable () throws -> T) async throws -> T {
        return try await withCheckedThrowingContinuation { (continuation) in
            self.submit { shouldRun in
                guard case shouldRun = NIOThreadPool.WorkItemState.active else {
                    continuation.resume(throwing: NIOThreadPoolError.ThreadPoolInactive())
                    return
                }
                
                do {
                    continuation.resume(returning: try body())
                } catch {
                    continuation.resume(throwing: error)
                }
            }
        }
    }
}

However, I was unable to do so because the init of NIOThreadPoolError.ThreadPoolInactive is internal.
It would be desirable for users to be able to use it conveniently in such cases by making the init of the error public.

[Lukasa]

This is a totally reasonable change, thanks!

[Lukasa]

@swift-server-bot test this please