func (e *SyncErrors) Append(err error) {
	e.mx.Lock()
	e.Errors = append(e.Errors, err)
	e.mx.Unlock()
}
Append appends given err.
package xerrors
import (
	"strings"
	"sync"
)
type Errors []error
func (e Errors) ErrorOrNil() error {
	if len(e) == 0 {
		return nil
	}
	return e
}
func (e Errors) Error() string {
	var s []string
	for _, err := range e {
		if err != nil {
			s = append(s, err.Error())
		}
	}
	return strings.Join(s, ". ")
}
type SyncErrors struct {
	mx sync.Mutex
	Errors
}