Use canonical error value for unchanged password errors

Author: dhui

credential.go

@@ -118,7 +118,7 @@ func (c *Credential) ChangePasswordWithConfigAndIP(config Config, oldPassword, n
 		return errors.New("Old password does not match existing password")
 	}
 	if subtle.ConstantTimeCompare([]byte(oldPassword), []byte(newPassword)) == 1 {
-		return errors.New("Password unchanged")
+		return ErrPasswordUnchanged
 	}
 	return c.ResetWithConfigAndIP(config, newPassword, ip)
 }

errors.go

@@ -0,0 +1,11 @@
+package passhash
+
+import (
+	"errors"
+)
+
+var (
+	// ErrPasswordUnchanged is used when a Credential.ChangePassword*() method is called with the same old and new
+	// password
+	ErrPasswordUnchanged = errors.New("Password unchanged")
+)