When a block is broken in a layout, we show a red message as the block description:

backdrop-broken_block

...but there is no hint whatsoever elsewhere. I would expect that red color to "propagate" back all the way forming some sort of trail that the site admin could follow. So there should be a site status error too that triggers the admin bar red badge to show:

backdrop-site_status-block_problems_indicators

...and then on the manage layouts page, the layout name (and perhaps the template icon too) should also be red:

backdrop-manage_layouts-block_problems_indicators

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/2320
GitHub Issue #: 
2320