throw error when input file cannot be opened

example.cpp

@@ -13,6 +13,9 @@ int main(int argc, char** argv)
    }
 
    std::fstream input_file_graph{argv[1]};
+   if (not input_file_graph.is_open()) {
+      throw std::runtime_error(std::string("Could not open file for reading: ") + argv[1]);
+   }
    ED::Graph const graph = ED::Graph::read_dimacs(input_file_graph);
 
    ED::Graph greedy_matching_as_graph{graph.num_nodes()};