[VFS] Use generic_category for errors generated from the VFS

Errors are generally checked in clients by comparing to the portable
error condition in `std::errc`, which will have the `generic_category`
(eg. `std::errc::no_such_file_or_directory`). While in practice these
are usually equivalent for the standard errno's, they are not in *all*
implementations. One such example is CentOS 7.

Differential Revision: https://reviews.llvm.org/D120299