Fix exceptions thrown from directory::copy

[bugdea1er]

According to the C++ standard:

When a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, an exception of type filesystem_error shall be thrown. For functions with a single path argument, that argument shall be passed to the filesystem_error constructor with a single path argument...

This fixes the exception thrown by directory::copy and adds tests for it