Struct multierror is not support Unwrap method

[viartos]

For correct working go 1.13 error chains (including errors.Is()) needs implementation of interface:

type wrapper interface {
	Unwrap() error
}

For example:
when i use github.com/gojek/heimdall and if at the time of request occurred context cancel, i am check it as if errors.Is(err, context.Canceled) {}, but it's not work because heimdall use valkyrie multierror that does't support method Unwrap.