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']);