GCC warning useless-cast

When compiling with gcc and `-Wuseless-cast`, this triggers a warning (or error with `-Werror`).
```
CLI/App.hpp:2273:24: error: useless cast to type ‘class CLI::FileError’ [-Werror=useless-cast]
 2273 |             throw CLI::FileError(std::move(fe));
      |       
```
Just `throw fe;` should be fine.