Don't mask previous failures.

src/main.cpp

@@ -192,12 +192,13 @@ int main(int argc, char *argv[])
 
                 if (int ret{}; (ret = it->get()) != EXIT_SUCCESS)
                 {
-                    return_code = ret;
+                    if (return_code == EXIT_SUCCESS)
+                    {
+                        return_code = ret;
+                    }
                 }
                 futurepool.erase(it);
             }
-
-            return EXIT_SUCCESS;
         }};
 
     const auto max_threads{