Do not wrap original err on fallback

[docbobo]

f you want to implement a conditional fallback, it's important that the original error is not wrapped when returned from the fallback. Otherwise it becomes impossible for the caller to distinguish e.g. ErrCircuitOpen from ErrTimeout.

[afex]

please provide an example of a conditional fallback and caller which you are having trouble implementing without this patch

[docbobo]

The following code works:

func ExampleError() {
    err := hystrix.Go("my_command", func() error {
        time.Sleep(2 * time.Minute)
        return nil
    }, nil)

    switch <-err {
    case hystrix.ErrTimeout:
        fmt.Println("Timeout")

    default:
        fmt.Println("Unknown error")
    }

    // Output: Timeout
}

However, adding a very simple fallback makes it fail:

func ExampleError() {
    err := hystrix.Go("my_command", func() error {
        time.Sleep(2 * time.Minute)
        return nil
    }, func(err error) {
        return err
    })

    switch <-err {
    case hystrix.ErrTimeout:
        fmt.Println("Timeout")

    default:
        fmt.Println("Unknown error")
    }

    // Output: Timeout
}

This seems to be wrong and I'd probably go even further, suggesting to get rid of all the fallback error wrapping, now that I've written this example.

Now where do I want to use this? In case of an error, I want to potentially return a previously cached result. If it's not available, I'd like to propagate the error to the outside of the operation. Depending on the particular result, I would like to retry the operation. For example, I'd want to retry on a max concurrency error after waiting a little. I do not want to apply the same logic on e.g. Circuit Closed. With the current error wrapping, there is no good way to tell those two cases apart - at least not when using a fallback function.