Expose NSError

[martinbonnin]

Expose the platform error in ApolloNetworkError:

    try {
      apolloClient.query(HeroNameQuery())
    } catch (e: ApolloNetworkException) {
      when((e.platformCause as? NSError)?.code) {
        NSURLErrorCannotFindHost -> handleCannotFindHost()
        NSURLErrorNetworkUnavailableReason -> handleNetworkUnavailable()
        else -> // fallback
      }
    }

Closes #3307