在Go中打印错误对象的正确格式说明符是什么：％s或％v？

Here is my program.

package main

import (
    "errors"
    "fmt"
)

func main() {
    a := -1
    err := assertPositive(a)
    fmt.Printf("error: %s; int: %d
", err, a)
    fmt.Printf("error: %v; int: %d
", err, a)
}

func assertPositive(a int) error {
    if a <= 0 {
        return errors.New("Assertion failure")
    }
    return nil
}

Here is the output.

error: Assertion failure; int: -1
error: Assertion failure; int: -1

In this program, it makes no difference whether I use %s or %v to print the error object.

I have two questions.

1. Is there any scenario while printing errors where it would make a difference for %s and %v?
2. What is the correct format specifier to use in this case?