* add `strerror` function
* move `getErrno` to `System.Errno`
* use `strerror` in `Show FileError`
* on node there's no access to `strerror`, so `strerror` just converts the number to string
Pragma once is supported by all compilers for the last ten years.
Better use it instead of include guards (which use different styles
in different files).
