add internal auth errors

internal/auth/errors.go

@@ -0,0 +1,13 @@
+package auth
+
+import "fmt"
+
+var (
+	ErrUserNotFound    = fmt.Errorf("user not found")
+	ErrInvalidPassword = fmt.Errorf("invalid password")
+	ErrInvalidEmail    = fmt.Errorf("invalid email")
+	ErrInvalidUsername = fmt.Errorf("invalid username")
+
+	ErrTokenIsMissing = fmt.Errorf("token is missing")
+	ErrInvalidToken   = fmt.Errorf("invalid token")
+)