Ref T13258. The general idea here is "if arc land prompted you and you hit 'y', you get a warning about it on the timeline".
This is similar to the existing warning about landing revisions in the wrong state and hitting "y" to get through that. See D18808, previously.
These warnings make it easier to catch process issues at a glance, especially because the overall build status is now more complicated (and may legally include some failures on tests which are marked as unimportant).
The transaction stores which builds had problems, but I'm not doing anything to render that for now. I think you can usually figure it out from the UI already; if not, we could refine this.
