attemptMap for Reactive Cocoa that deals with `throws`

attemptMap.swift

func attemptMap<T, U>(operation: T throws -> U) -> ReactiveCocoa.Signal<T, NSError> -> ReactiveCocoa.Signal<U, NSError> {
    return { signal in
        return Signal { observer in
            signal.observe(next: { value in
                do {
                    sendNext(observer, try operation(value))
                }
                catch {
                    sendError(observer, error as NSError)
                }
                }, error: { error in
                    sendError(observer, error)
                }, completed: {
                    sendCompleted(observer)
                }, interrupted: {
                    sendInterrupted(observer)
            })
        }
    }
}

The only thing about this is because ErrorType is so loosey goosey we have to bridge to NSError, and we lose type information 😞