https://chromium-review.googlesource.com/c/545116/

^^ Looks like we may be cancelling the wrong pre-cq slaves sometimes as well (this is more than just messaging).
In this case
- we launched some pre-cq slaves for PS7
- PS8 was uploaded, CQ:+1ed.
- We didn't post any message saying we're trying for PS8
- then we cancelled old builds (for PS7?)
- posted to PS8 the confusing message that pre-cq-slaves failed
- Removed the CQ bit from PS8 <--- this is a bug.

User action was needed at this point to get PS8 back in the queue.

This is blocked on coalescing all PreCQ email batches into a single email. Once we've done that, we can be better about sending these kinds of emails. That's tracked in  crbug.com/842074  .