Fixes AttributeError when using storage.exists() on a non-existing file.
Fixes AttributeError when using storage.exists() on a non-existing file.

RussellLuo/mongoengine,ms5/mongoengine,larsbutler/mongoengine,blaggacao/mongoengine,ProgressivePlanning/mongoengine,cogniteev/mongoengine,jimmyshen/mongoengine,Davidrjx/mongoengine,muhammad-ammar/mongoengine,elephanter/mongoengine,starsirius/mongoengine,MongoEngine/mongoengine,nathanial/mongoengine,touilleMan/mongoengine,yarneo/mongoengine,MRigal/mongoengine,khallaghi/mongoengine,cogniteev/mongoengine,9nix00/mongoengine,gavincabbage/mongoengine,sourcelair/mongoengine,Davidrjx/mongoengine,reallistic/mongoengine,ConsumerPhysics/mongoengine,prijindal/mongoengine,MakerReduxCorp/mongoengine,noirbizarre/mongoengine,nicolascine/mongoengine,iandd0824/mongoengine,gmacon/mongoengine,MakerReduxCorp/mongoengine,losintikfos/mongoengine,DavidBord/mongoengine,Multiposting/mongoengine,eli-b/mongoengine,Catstyle/mongoengine,WingGao/mongoengine