Improve code on loadMetadata() to verify if class exists

Improve code on loadMetadata() to verify if class exists, avoid a later warning when calling class_parents()

Instead throws a \RuntimeException if class doesn't exists.

Before changing code, just after add test:
```
1) Doctrine\Tests\Common\Persistence\Mapping\ClassMetadataFactoryTest::testGetMetadataForAbsentClass
class_parents(): Class Doctrine\Tests\Common\Persistence\Mapping\AbsentClass does not exist and could not be loaded

/home/www/common/lib/Doctrine/Common/Persistence/Mapping/RuntimeReflectionService.php:38
/home/www/common/lib/Doctrine/Common/Persistence/Mapping/AbstractClassMetadataFactory.php:257
/home/www/common/lib/Doctrine/Common/Persistence/Mapping/AbstractClassMetadataFactory.php:281
/home/www/common/lib/Doctrine/Common/Persistence/Mapping/AbstractClassMetadataFactory.php:212
/home/www/common/tests/Doctrine/Tests/Common/Persistence/Mapping/ClassMetadataFactoryTest.php:44
```