Go の `fmt` パッケージが印刷時に `String()` よりも `Error()` を優先するのはなぜですか?

3. If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).
4. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

package main

import "fmt"

type Person struct {
    Name string
    Age  int
}

func (p *Person) String() string {
    return fmt.Sprintf("%v (%v years)", p.Name, p.Age)
}

func (p *Person) Error() string {
    return fmt.Sprintf("Failed")
}

func main() {
    a := &Person{"Arthur Dent", 42}
    z := &Person{"Zaphod Beeblebrox", 9001}
    fmt.Println(a, z)
}

Failed Failed