Implement data check/warning in the authoring tool

[JoeGermuska]

It's clear that we can't impose validation which stops showing timelines, because there is too much risk of "breaking" timelines which were already published.

However, we should be able to do something that runs in the "authoring tool" that can display warnings near the preview, such as "column X was not found in your spreadsheet" or "invalid date in for slide [title], that slide was skipped"

[scott2b]

general data validation should account for discussion in #310 closed in lieu of current issue