Make it clear this is an internal error

src/launchpad.cpp

@@ -214,7 +214,7 @@ std::string launchpad::build_full_url(const std::string& endpoint) const {
     } else if (endpoint != "") {
         return service_root + "/" + api_version + "/" + endpoint;
     }
-    std::cerr << "endpoint is empty" << std::endl;
+    std::cerr << "Internal error: endpoint is empty" << std::endl;
     return "";
 }