Fix in.Rcode type for fmt.Errorf() (#542)

in.Rcode returns an int.

This resolves build issue:

	./va.go:845:80: fmt.Errorf format %q has arg in.Rcode of wrong type int

Author: paulwouters

va/va.go

@@ -842,7 +842,7 @@ func (va VAImpl) getTXTEntry(name string) ([]string, error) {
 	}
 
 	if in.Rcode != dns.RcodeSuccess {
-		return nil, fmt.Errorf("DNS lookup for %q returned an unsuccessful response: %q", name, in.Rcode)
+		return nil, fmt.Errorf("DNS lookup for %q returned an unsuccessful response: %d", name, in.Rcode)
 	}
 
 	for _, record := range in.Answer {