fixing #276

#276 still exists after previous fix 99c915892812aa35f0ce396d7014b9d104e1ba3c . doctrine:mapping:convert --from-database still throws an exception about a non-existing class. The class metadata is generated by Doctrine\ORM\Mapping\Driver\DatabaseDriver so there might not be an actual entity class file for it. But Gedmo\Tree\TreeListener would try to load the class for its reflection info, which causes the exception. This should not be expected as tree behaviors should only apply to existing entities.

FYI, Here's the call stack generated by xdebug before the exception happens. 

```
sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php.Doctrine\ORM\Mapping\ClassMetadataInfo->getReflectionClass : lineno 516() sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php at line 516	
sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Tree/Mapping/Driver/Annotation.php.Gedmo\Tree\Mapping\Driver\Annotation->readExtendedMetadata : lineno 109() sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Tree/Mapping/Driver/Annotation.php at line 109	
sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Mapping/ExtensionMetadataFactory.php.Gedmo\Mapping\ExtensionMetadataFactory->getExtensionMetadata : lineno 95() sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Mapping/ExtensionMetadataFactory.php at line 95	
sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Mapping/MappedEventSubscriber.php.Gedmo\Mapping\MappedEventSubscriber->loadMetadataForObjectClass : lineno 191() sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Mapping/MappedEventSubscriber.php at line 191	
sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Tree/TreeListener.php.Gedmo\Tree\TreeListener->loadClassMetadata : lineno 262() sf2-blue/vendor/gedmo-doctrine-extensions/lib/Gedmo/Tree/TreeListener.php at line 262	
sf2-blue/vendor/doctrine-common/lib/Doctrine/Common/EventManager.php.Doctrine\Common\EventManager->dispatchEvent : lineno 64() sf2-blue/vendor/doctrine-common/lib/Doctrine/Common/EventManager.php at line 64	
sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php.Doctrine\ORM\Mapping\ClassMetadataFactory->loadMetadata : lineno 317() sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php at line 317	
sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php.Doctrine\ORM\Mapping\ClassMetadataFactory->getMetadataFor : lineno 177() sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php at line 177	
sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php.Doctrine\ORM\Mapping\ClassMetadataFactory->getAllMetadata : lineno 124() sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php at line 124	
sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Tools/Console/Command/ConvertMappingCommand.php.Doctrine\ORM\Tools\Console\Command\ConvertMappingCommand->execute : lineno 129() sf2-blue/vendor/doctrine/lib/Doctrine/ORM/Tools/Console/Command/ConvertMappingCommand.php at line 129	
sf2-blue/vendor/symfony/src/Symfony/Bundle/DoctrineBundle/Command/Proxy/ConvertMappingDoctrineCommand.php.Symfony\Bundle\DoctrineBundle\Command\Proxy\ConvertMappingDoctrineCommand->execute : lineno 50() sf2-blue/vendor/symfony/src/Symfony/Bundle/DoctrineBundle/Command/Proxy/ConvertMappingDoctrineCommand.php at line 50	
sf2-blue/vendor/symfony/src/Symfony/Component/Console/Command/Command.php.Symfony\Component\Console\Command\Command->run : lineno 224() sf2-blue/vendor/symfony/src/Symfony/Component/Console/Command/Command.php at line 224	
sf2-blue/vendor/symfony/src/Symfony/Component/Console/Application.php.Symfony\Component\Console\Application->doRun : lineno 194() sf2-blue/vendor/symfony/src/Symfony/Component/Console/Application.php at line 194	
sf2-blue/vendor/symfony/src/Symfony/Bundle/FrameworkBundle/Console/Application.php.Symfony\Bundle\FrameworkBundle\Console\Application->doRun : lineno 75() sf2-blue/vendor/symfony/src/Symfony/Bundle/FrameworkBundle/Console/Application.php at line 75	
sf2-blue/vendor/symfony/src/Symfony/Component/Console/Application.php.Symfony\Component\Console\Application->run : lineno 118() sf2-blue/vendor/symfony/src/Symfony/Component/Console/Application.php at line 118	
sf2-blue/app/console.{main} : lineno 22() sf2-blue/app/console at line 22	

```