Use fs_err for improved file system error messages

[Luke Frisken]

Currently error messages triggered in std::fs package lack important information required to debug the problem, namely the path to the files involved.

Replacing the use of std::fs with fs_err wrapper should provide us with more descriptive error types, reducing time required to debug problems involved with storage, and making it easier to diagnose problems in deployment.

If you've ever had to debug an error that occurs in this package (I've spent time on this today) you'll know what I mean! I think this will result in some extra allocations for the paths, and a small amount of extra indirection, but I'm guessing probably that's chicken feed compared to the rest of the system.