FR: return identifiable errors in iid package

InstanceID client returns a formatted error message: https://github.com/firebase/firebase-admin-go/blob/master/iid/iid.go#L88

Caller cannot distinguish the error and take action. Errors should be public and returned error should be the error itself.

```go
if err, ok := errorCodes[resp.Status]; ok {
	return err
}
```