"Don't consider all errors as warnings"

Now to Sergii, and the errors-as-values concept from Module 1, Lesson 5, as it showed up in your review. On PR #401 (partial results for GetFlights), you'd treated failures uniformly, folding everything into "warnings." Sergii's comment on application/flights/GetFlights.cs:

This is the typed-errors point from the foundations lesson, made concrete. Collapsing every failure into one generic "warning" throws away the distinction the domain cares about. Is it a flight that genuinely doesn't exist? A timeout? A permissions problem? Once they're all "a warning," nobody downstream can tell, and nobody can respond differently. Sergii's fix is to lift the distinction into the type system: a FlightNotFound type, produced where the failure actually happens (the DB layer), and pattern-matched at the application layer to decide what it means.

There was a second comment on the same PR, on GetFlightsResponse.cs, and it's a layering point hiding in a naming complaint:

Response is a presentation word: it belongs to the API layer, the thing that talks HTTP. You'd introduced it in the application layer. That leaks presentation vocabulary inward, which is both a ubiquitous-language smell (the wrong word for the layer) and a layering smell (an inner layer reaching for an outer layer's concept). Sergii's fix expresses the idea where it belongs: a domain-level Result extension, ResultWithWarnings, so the concept "a success that carries non-fatal warnings" lives in the domain's own language.

The convention doc (D5/E2, distilled from Nick and Sergii across #272/#251) states the endpoint-side rule that follows:

Sergii's feedback is a good example of the meta-note in the dossier: your replies to it were solid reasoning, you just arrived at the typed-error design in review rather than at design time. The habit to build is asking, before you write the happy path: what are the distinct ways this can fail, and does each deserve its own type? The next lesson is Nick's, and it's the anaemic-domain trap you learned in the abstract, caught in your actual code.