Initial, composed implementation of `Publisher.withLatestFrom`’s handling of error events.

with_latest_from_failure_events.swift

// **Scenario one**: `withLatestFrom`’s argument erroring out first.

struct SomeError: Error {}

let first = PassthroughSubject<Int, SomeError>()
let second = PassthroughSubject<String, SomeError>()
var cancellables = Set<AnyCancellable>()

first
	.withLatestFrom(second) // Our composed operator implementation of `withLatestFrom`.
	.sink(receiveCompletion: { print($0) }, receiveValue: { print($0) })
	.store(in: &cancellables)

first.send(1)
second.send("one")
first.send(2)
second.send("two")
second.send(completion: .failure(.init()))

// Outputs:
//
// ```
// one
// failure(SomeError())
// ```

// **Scenario two**: Upstream erroring out first.

// …

first.send(1)
second.send("one")
first.send(2)
second.send("two")
first.send(completion: .failure(.init())) // `first` is subbed in for `second` here.

// Outputs:
//
// ```
// one
// failure(SomeError())
// ```