You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: errors.go
    path of file: ./repos/teller/vendor/github.com/urfave/cli/v2
    
    the code of the file until where you have to start completion: package cli

import (
	"fmt"
	"io"
	"os"
	"strings"
)

// OsExiter is the function used when the app exits. If not set de
    
func (e *errRequiredFlags) Error() string {
	numberOfMissingFlags := len(e.missingFlags)
	if numberOfMissingFlags == 1 {
		return fmt.Sprintf("Required flag %q not set", e.missingFlags[0])
	}
	joinedMissingFlags := strings.Join(e.missingFlags, ", ")
	return fmt.Sprintf("Required flags %q not set", joinedMissingFlags)
}