feat: new general and auth errors;

feat: NewPointer helper function in utils;
refactor: length validation in auth models

backend/internal/errors/auth.go

@@ -24,6 +24,7 @@ import (
 var (
 	ErrUnauthorized = errors.New("User is not authorized")
 	ErrUsernameTaken = errors.New("Provided username is already in use")
+	ErrUserNotFound = errors.New("User was not found")
 	ErrInvalidCredentials = errors.New("Invalid username, password or TOTP code")
 	ErrInvalidToken = errors.New("Token is invalid or expired")
 	ErrServerError = errors.New("Internal server error")

backend/internal/errors/general.go

@@ -24,4 +24,5 @@ import (
 var (
 	ErrNotImplemented = errors.New("Feature is not implemented")
 	ErrBadRequest = errors.New("Bad request")
+	ErrForbidden = errors.New("Access is denied")
 )