Create custom validation-errors in Neos.Flow

Example.fusion

// render all validation errors of current request
renderer = afx`
    <div @if.hasErrors={request.internalArguments.__submittedArgumentValidationResults.flattenedErrors}>
        ERORR
        <ul>
            <Neos.Fusion:Loop items={request.internalArguments.__submittedArgumentValidationResults.flattenedErrors} itemKey="path" itemName="errors">
            <Neos.Fusion:Loop items={errors} itemName="error">
            <li>{path} - {error}</li>
            </Neos.Fusion:Loop>
            </Neos.Fusion:Loop>
        </ul>
    </div>
`

ExampleController.php

public function testAction()
{
    // create result
    $result = new Result();
    $result->addError( new Error("a generic error") );
    $result->forProperty('foo.bar')->addError(new Error( "a path error") );
    
    // fake ValidationResult for the request we are forwarding to
    $arguments = [];
    $arguments['__submittedArguments'] = [];
    $arguments['__submittedArgumentValidationResults'] = $result;

    // forward to "other" action but with the defined errors
    $this->forward('action', 'Example', 'Vendor.Site', $arguments);
}