better handling of comments

ml_io.cpp

@@ -10,7 +10,7 @@ std::string read_file_contents(const std::string &filename) {
 	std::ifstream f;
 	f.open(filename.c_str());
 	if (!f)
-		throw std::runtime_error("could not open file");
+		throw std::runtime_error("could not open file (" + filename + ")");
 
 	f.seekg(0, std::ios::end);
 	std::string contents;