Improve Shopify sync error logging

Log clear hint for 403 (IP whitelist), 401, 429, 5xx.
Commands exit with failure and print error when API returns no data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Console/Commands/ShopifyMap.php

@@ -16,6 +16,11 @@ class ShopifyMap extends Command
         $dryRun = $this->option('dry-run');
         $variants = $shopify->getAllVariants();
 
+        if (empty($variants)) {
+            $this->error('Keine Varianten von Shopify erhalten — API-Aufruf fehlgeschlagen (Details im Log).');
+            return self::FAILURE;
+        }
+
         $this->info('Shopify-Varianten geladen: ' . count($variants));
 
         $mapped = 0;