Three options exist for a bad block: raise, render nothing, or render the error.
Raising takes down a whole page for one typo. Rendering nothing is worse — the author ships a page with a hole in it and never learns. Rendering the error keeps the page readable and puts the message where the person who can fix it will see it.
The same rule holds for data: a src file that has moved shows a missing-file card, so the page tells you the chart is stale rather than quietly drawing an empty bar.