Bug - Dashboard Cells - Change text that displays when a user has data but the system is still loading the visualization

## Problem
Present day, if a user has a dashboard cell configured, yet the data hasn't been rendered yet, the error message displayed is "Looks like you don't have any data, it'd be a lot cooler if you did." This error message doesn't make sense as it implies that the user has an action to take when in fact they do not. The screen just hasn't loaded their data in the visualization yet.

## Proposal
Update the message that displays to the user when a dashboard cell is still rendering the data in their visualization. Consult design as needed. Perhaps have a standard message that displays across all visualizations in our system when the visualization is in the process of rendering the user's data. @mavarius