Function merge_peek_errors 

fn merge_peek_errors(error1: PeekError, error2: PeekError) -> PeekError

Merge two PeekErrors into the one we report.

A row-iteration-limit error only wins against another one, and then the smaller limit wins so the choice does not depend on which worker answered first. Any other error outranks it: a query that both overran the limit and failed for a real reason should report the real reason.