add validation error type, fix tests, add number tests

error.go

@@ -0,0 +1,50 @@
+package validate
+
+import "fmt"
+
+// Validation error.
+var (
+	Err = Error{}
+)
+
+// Error represents a validation error.
+type Error struct {
+	Message string
+	Data    []any
+}
+
+// Error retrieves the message of a validation Error.
+// If it has Data, the message will be formatted.
+func (e Error) Error() string {
+	if len(e.Data) > 0 {
+		return fmt.Sprintf(e.Message, e.Data...)
+	}
+	return e.Message
+}
+
+// Is determines whether the Error is an instance of the target.
+// https://pkg.go.dev/errors#Is
+//
+// If the target is a validation error and specifies a message, this function returns true if the messages match.
+// If the target is an empty validation error, this function always returns true.
+func (e Error) Is(target error) bool {
+	if t, ok := target.(Error); ok {
+		return t.Message == e.Message || t.Message == ""
+	}
+	return false
+}
+
+func (e Error) With(value any) Error {
+	if e.Data == nil {
+		e.Data = []any{}
+	}
+	e.Data = append(e.Data, value)
+	return e
+}
+
+// NewError creates a new validation error.
+func NewError(message string) Error {
+	return Error{
+		Message: message,
+	}
+}