message: Throw exceptions if file not found
diff: @@ -94,7 +94,10 @@ template< typename Graph_t, typename Filtration_value, typename Vertex_handle >
  Graph_t read_graph(std::string file_name) {
  std::ifstream in_(file_name.c_str(), std::ios::in);
  if (!in_.is_open()) {
  - std::cerr << "Unable to open file " << file_name << std::endl;
  + std::string error_str("read_graph - Unable to open file ");
  + error_str.append(file_name);
  + std::cerr << error_str << std::endl;
  + throw std::invalid_argument(error_str);
  }
  typedef std::pair< Vertex_handle, Vertex_handle > Edge_t;
  @@ -308,10 +311,10 @@ void read_persistence_intervals_and_dimension(std::string const& filename, Outpu
  std::ifstream in(filename);
  if (!in.is_open()) {
  -#ifdef DEBUG_TRACES
  - std::cerr << "File \"" << filename << "\" does not exist.\n";
  -#endif // DEBUG_TRACES
  - return;
  + std::string error_str("read_persistence_intervals_and_dimension - Unable to open file ");
  + error_str.append(filename);
  + std::cerr << error_str << std::endl;
  + throw std::invalid_argument(error_str);
  }
  while (!in.eof()) {
  