fix: surface parsing errors instead of silently discarding them

Return errors for required fields (ID, price) and log warnings for
optional fields (title, description, unit price) across all site
extractors instead of silently discarding them with _ =.

Closes #24
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sites/duckduckgo/page.go

@@ -2,9 +2,10 @@ package duckduckgo
 
 import (
 	"fmt"
-	"gitea.stevedudenhoeffer.com/steve/go-extractor"
 	"io"
 	"log/slog"
+
+	"gitea.stevedudenhoeffer.com/steve/go-extractor"
 )
 
 type SearchPage interface {
@@ -44,13 +45,19 @@ func extractResults(doc extractor.Node) ([]Result, error) {
 		titles := n.Select("h2")
 
 		if len(titles) != 0 {
-			r.Title, _ = titles[0].Text()
+			r.Title, err = titles[0].Text()
+			if err != nil {
+				slog.Warn("failed to get result title", "err", err)
+			}
 		}
 
 		descriptions := n.Select("span > span")
 
 		if len(descriptions) != 0 {
-			r.Description, _ = descriptions[0].Text()
+			r.Description, err = descriptions[0].Text()
+			if err != nil {
+				slog.Warn("failed to get result description", "err", err)
+			}
 		}
 
 		res = append(res, r)