repeat_row: Make subquery guard aware of negative diffs

[ggevay]

This is the 4. PR in the repeat_row productionization work stream. Resolve SQL-164.

This resolves #32978 (comment). It was not important originally, but now that we are productionizing repeat_row, the original error message would become confusing: it would say that "more than one record produced in subquery", when in fact there was a negative number of rows.

I didn't add a test, because I wasn't able to find a query where this error deterministically happens. If I just naively put a repeat_row(-1) inside a subquery, then sometimes this error happens, but sometimes a Distinct gives the negative accumulation error message.

[frankmcsherry]

Seems fine. Personally, the error seems most accurately "Error: not exactly one row", and the different variants for negative vs more than one may not pay dividends. But .. not opposed other than wrt the structured error bloat.

[ggevay]

The reason for making the negative case its own error is that we'd like to handle this case specially: The error message should say something like "this is an internal error, unless you are using repeat_row". Also, we might want to create an incident for this one if repeat_row is not enabled in the user's env, but never create an incident for the >1 case. (I'll do the error message tweak in a separate PR, together with all the other negative accumulation error msgs.)