Mapping validation does not validate targetDocument

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Bug Report

Q	A
BC Break	maybe
Version	2.5.x

Summary

Validity of EmbedOne/EmbedMany/ReferenceOne/ReferenceMany with targetDocument is not confirmed.

Current behavior

Running doctrine:mongodb:mapping:info or just warming up the ClassMetadata does not detect references within EmbedOne/EmbedMany/ReferenceOne/ReferenceMany to targetDocuments that cannot be resolved.

How to reproduce

/** @ODM\Document */
class InvalidTargetDocumentTestDocument
{
    /**
     * @ODM\Id
     *
     * @var string|null
     */
    public $id;

    /**
     * @ODM\ReferenceOne(targetDocument=Doctrine\ODM\MongoDB\Tests\Functional\SomeInvalidClass::class)
     *
     * @var object|null
     */
    public $reference;
}

Expected behavior

ClassMetadata generation should fail or doctrine:mongodb:mapping:info should display an error.