[assbin] Fail if file could not be opened

Fail instead of returning empty scene.

Partially addresses #3888.

code/AssetLib/Assbin/AssbinLoader.cpp

@@ -671,7 +671,7 @@ void AssbinImporter::ReadBinaryScene(IOStream *stream, aiScene *scene) {
 void AssbinImporter::InternReadFile(const std::string &pFile, aiScene *pScene, IOSystem *pIOHandler) {
     IOStream *stream = pIOHandler->Open(pFile, "rb");
     if (nullptr == stream) {
-        return;
+        throw DeadlyImportError("ASSBIN: Could not open ", pFile);
     }
 
     // signature