writer.go
func (f *FileLogger) E(format string, v ...interface{}) {
	f.Error(format, v...)
}
func (f *FileLogger) E(format string, v ...interface{}) {
	f.Error(format, v...)
}
func ( f * FileLogger ) E ( format string , v ... interface { } ) { f . Error ( format , v ... ) }
// same with Error()
same with Error ()