Improve document generation error message when mapping / configuration is not available
Improve document generation error message when mapping / configuration is not available

Closes #150

castarco/DoctrineMongoDBBundle,tomasbro/DoctrineMongoDBBundle,stof/DoctrineMongoDBBundle,Soullivaneuh/DoctrineMongoDBBundle,alcaeus/DoctrineMongoDBBundle,sarcher/DoctrineMongoDBBundle,juchi/DoctrineMongoDBBundle,castarco/DoctrineMongoDBBundle,jahller/DoctrineMongoDBBundle,juchi/DoctrineMongoDBBundle,tomasbro/DoctrineMongoDBBundle,0mars/DoctrineMongoDBBundle,malarzm/DoctrineMongoDBBundle,alcaeus/DoctrineMongoDBBundle,0mars/DoctrineMongoDBBundle,doctrine/DoctrineMongoDBBundle,malarzm/DoctrineMongoDBBundle,Soullivaneuh/DoctrineMongoDBBundle,jahller/DoctrineMongoDBBundle