add all validator, simplify error messages

chars.go

@@ -1,17 +1,22 @@
 package validate
 
 import (
-	"fmt"
+	"errors"
 	"strings"
 )
 
+// Validation error.
+var (
+	ErrDisallowedChars = errors.New("contains disallowed characters")
+)
+
 // Chars validates whether a string contains only allowed characters.
 func Chars(allow string) func(string) error {
 	return func(value string) error {
 		rs := []rune(value)
 		for _, r := range rs {
 			if !strings.ContainsRune(allow, r) {
-				return fmt.Errorf("Contains disallowed characters")
+				return ErrDisallowedChars
 			}
 		}
 		return nil