Reduce stacktrace of VendorListService (prebid#589)

Author: AndriyPavlyuk

src/main/java/org/prebid/server/privacy/gdpr/vendorlist/VendorListService.java

@@ -286,7 +286,9 @@ private Future<Void> saveToFile(String content, int version) {
      * Handles errors occurred while HTTP request or response processing.
      */
     private static Future<Void> failResponse(Throwable exception, int version) {
-        logger.warn("Error fetching vendor list via HTTP for version {0}", exception, version);
+        logger.warn("Error fetching vendor list via HTTP for version {0} with message: {1}",
+                version, exception.getMessage());
+        logger.debug("Error fetching vendor list via HTTP for version {0}", exception, version);
         return Future.failedFuture(exception);
     }