std::variant
In the previous exercise we used std::optional to indicate failure.
But how about a more detailed error message?
Enter template class std::variant.
We could use std::variant to either hold a valid value or a detailed error.
DIY
Refactor function differentiate to have return type std::variant<double, std::string> and
once again make the tests pass.
The variant should either be a valid value or an error string.
Refactor the code to use std::variant
