Support for path mapping of error messages

[pepamartinec]

This enables validators to map error message to form element children. Its equivalent of Symfonys ConstraintViolationBuilder->atPath().

PHP example:

class MyValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {
        ...

        if ($isInvalid) {
            $this->context
                ->buildViolation($constraint->message)
                ->atPath('payment')
                ->addViolation();
        }
    }
}

JS counterpart

window.MyValidator = function() {
    this.validate = function (value) {
        ...

        if (isInvalid) {
            let error = new FpJsFormError(this.message);
            error.atPath = 'payment';

            return [error];
        }

        return [];
    }
};

The validator is now expected to return instances of FpJsFormError. If string is returned, it is automatically converted to FpJsFormError to keep backward compatibility with existing validators.

Internally, implementation keeps concept of error sourceId. It just recursively delegates error messages to element children.

[66Ton99]

@Joker806 It looks messy. Can you update it?

[66Ton99]

Closing this in favor of #178, which ports the error path mapping behavior to current master with focused Jest coverage, docs, and green CI. Thanks for the original implementation and context.