check file attributes before reading

[todo]

tan/src/reader/reader.cpp

Lines 19 to 24 in 12ad011

	// TODO: check file attributes before reading
	_filename = filename;
	std::ifstream ifs(filename);
	// read the whole file at once
	std::string content((std::istreambuf_iterator<char>(ifs)), (std::istreambuf_iterator<char>()));
	// count the number of lines

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This issue was generated by todo based on a TODO comment in 12ad011 when #6 was merged. cc @tjysdsg.

[tjysdsg]

Since ifstream will throw if it cannot open a file, there is no need to check beforehand.