Reduce repeated validation boilerplate and improve error reporting

[edunzer]

Many sequential if (...) return returnErrorWrapper(...) checks are fine, but hard to maintain and don’t tell which field failed beyond text.

How to fix: Use a List<String> errors and collect all validation errors, returning one wrapper containing all messages joined by newline. This improves Flow UX and reduces back-and-forth runs.

File: ScheduleHoursDistributor.cls (generateCategoryScheduleExceptions)