GetMongoConnection() doens't return error in failure state

File code.fbi.h-da.de/danet/gosdn/controller/nucleus/database/mongo-connection.go.

Function GetMongoConnection() doesn't return an error if one is found, it just prints it.

Proposed solution:

// GetMongoConnection Retrieves a client to the MongoDB.
func GetMongoConnection() (*mongo.Client, context.Context, context.CancelFunc, error) {
	mongoConnection := config.DatabaseConnection
	ctx, cancel := context.WithTimeout(context.Background(), connectTimeout*time.Second)

	client, err := mongo.Connect(ctx, options.Client().ApplyURI(mongoConnection))
	if err != nil {
		log.Printf("Failed to create client: %v", err)
		return client, ctx, cancel, err
	}

	// Force a connection to verify our connection string
	err = client.Ping(ctx, nil)
	if err != nil {
		log.Printf("Failed to connect to database: %v\n", err)
		return client, ctx, cancel, err
	}

	return client, ctx, cancel, nil
}

This leads to a lot of changes in storage code.