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: mount_errors.go
    path of file: ./repos/docker-ce/components/cli/vendor/github.com/moby/sys/mount
    
    the code of the file until where you have to start completion: //go:build !windows
// +build !windows

package mount

import "strconv"

// mountError records an error from mount or unmount operation
type mountError struct {
	op             string
	source, target string
	flags          uintptr
	data           string
	err   
    
func (e *mountError) Error() string {
	out := e.op + " "

	if e.source != "" {
		out += e.source + ":" + e.target
	} else {
		out += e.target
	}

	if e.flags != uintptr(0) {
		out += ", flags: 0x" + strconv.FormatUint(uint64(e.flags), 16)
	}
	if e.data != "" {
		out += ", data: " + e.data
	}

	out += ": " + e.err.Error()
	return out
}