sanity-check DI configuration

i installed dev-master of the mongo bundle in a current symfony standard edition without adding any configuration. the output was 

    [Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]
    Unable to replace alias "doctrine_mongodb.odm.document_manager" with "doctrine.odm.mongodb.document_manager".  

    [Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]
    The service definition "doctrine_mongodb.odm.document_manager" does not exist. 

with this change, we get the more telling message

    [Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]
    You need to define at least one entry in document_managers