add all validator, simplify error messages

uuid.go

@@ -5,13 +5,18 @@ import (
 	"regexp"
 )
 
+// Validation error.
+var (
+	ErrInvalidUUID = errors.New("invalid UUID")
+)
+
 var uuidRegexp = regexp.MustCompile("^[a-f0-9]{8}(-[a-f0-9]{4}){3}-[a-f0-9]{12}$")
 
 // UUID validates a UUID string.
 // The UUID must be formatted with separators.
 func UUID(value string) error {
 	if !uuidRegexp.MatchString(value) {
-		return errors.New("Invalid UUID")
+		return ErrInvalidUUID
 	}
 
 	return nil