Could a single error message print first on the report, if the school has Unverified Absence codes in ATT.AL (I don't know if it would be necessary for Period absences?) and they are older than two weeks? 30 days?

I don't think the error has to specify the student(s) or date(s). I was imagining something like this: "Unverified Absence code(s) in the All Day field exist and are older than fourteen days. Run the Students with Unverified Absences report or load the Attendance Management page for more details."