Recover failed conversion workers and validate Nitro downloads correctly

src/lib/services/furni-import.ts

@@ -750,7 +750,7 @@ export async function importSingleFurni(params: {
 		!existsSync(/*turbopackIgnore: true*/ nitroPath)
 	) {
 		const nitroUrl = `${sourceNitroBaseUrl}/${encodeURIComponent(safeNitroName)}.nitro`;
-		const dl = await downloadFile(nitroUrl, nitroPath, { validate: "png" });
+		const dl = await downloadFile(nitroUrl, nitroPath, { validate: "nitro" });
 		if (dl.ok) nitroDownloadOk = true;
 	}
 
@@ -843,7 +843,10 @@ export async function importSingleFurni(params: {
 			warnings,
 			error: isUpdate
 				? "No .nitro file could be created; existing furniture was preserved"
-				: "Import rolled back: no .nitro file could be created",
+				: `Import rolled back: no .nitro file could be created. ${warnings
+						.filter((warning) => /conversion|download|SWF/.test(warning))
+						.slice(-3)
+						.join(". ")}`,
 		};
 	}