How can I manually return or throw a validation error/exception in Laravel?

As of laravel 5.5, the ValidationException class has a static method withMessages that you can use:

$error = \Illuminate\Validation\ValidationException::withMessages([
   'field_name_1' => ['Validation Message #1'],
   'field_name_2' => ['Validation Message #2'],
]);
throw $error;

I haven't tested this, but it should work.

Update

The message does not have to be wrapped in an array. You can also do:

use Illuminate\Validation\ValidationException;

throw ValidationException::withMessages(['field_name' => 'This value is incorrect']);

Laravel <= 6.2 this solution worked for me:

$validator = Validator::make([], []); // Empty data and rules fields
$validator->errors()->add('fieldName', 'This is the error message');
throw new ValidationException($validator);

Simply return from controller:

return back()->withErrors('your error message');

---

or:

throw ValidationException::withMessages(['your error message']);