text: Title: Better warning message for Vis and VisList
  Body: We should display a better warning message when the user specifies an intent that indicates more than one item, but puts it into Vis instead of VisList.
  
  For example, if I do: 
  ```
  Vis(["AverageCost","SATAverage", "Geography=?"],df)
  ```
  ![image](https://user-images.githubusercontent.com/5554675/96357486-1ba70d80-112f-11eb-8ad9-66f570625fd7.png)
  The error that I get is not very interpretable. We should warn users that the intent should be used with a VisList as shown in the screenshot below. If possible, we might even map the intent down to a VisList so that we display something to the user.
  
  ![image](https://user-images.githubusercontent.com/5554675/96357500-6b85d480-112f-11eb-8167-bcd40fd07379.png)
  