Show "results not available" on public dashboard when not available yet

[jezdez]

Issue Summary

As a short term fix for #1194 as stated by @arikfr in #2823 (review) we should show a message on public dashboard widgets that don't have any results on the server side yet.

The long term solution of getting fresh results on public dashboards requires solving #2825 first.

Steps to Reproduce

1. Go to a public dashboard with a widget whose query results haven't been processed yet
2. See the dashboard crash as documented in Shared dashboard crashes if one of the queries has no results yet #1194.

This ticket is about providing a short term solution so the dashboard doesn't return a 500 error.

Technical details:

• Redash Version: 5.0.0
• Browser/OS: Firefox 62/macOS 10.13
• How did you install Redash: Docker

[jezdez]

@kocsmy Is this something we can help with implementation wise?

[arikfr]

#3659 solves this.

[jezdez]

@arikfr OMG!!