R1:18b444e427b0

When queries overheat, raise an exception

Summary:
Ref T13259. Currently, queries set a flag and return a partial result set when they overheat. This is mostly okay:

- It's very unusual for queries to overheat if they don't have a real viewer.
- Overheating is rare in general.
- In most cases where queries can overheat, the context is a SearchEngine UI, which handles this properly.

In T13259, we hit a case where a query with an omnipotent viewer can overheat: if you have more than 1,000…