Improve error message for readFile/writeFile

[dom96]

Seeing the following:

cannot open: /home/blah/foo.txt

Is vague. It would be better to include more detailed info like:

cannot open: /home/blah/foo.txt. Permission Denied.

[capocasa]

Surprisingly nontrivial. So C fopen uses a global variable errno for that information, which is implemented as a thread local or funciton call when C threads are enabled by compiler flag on modern system, but not on all systems.

So I would suggest to change the Nim open wrapper (but not the fopen wrapper) to

(1) If Nim is compiled with threads=off, assume it's a global, look up the meaning, and add it to the exception, and also add the error code itself to the exception and create a list of constants to represent them
(2) if Nim is compiled with threads=on, implement whatever is in the errnoC macro i Nim, possibly platform specific. This is going to require a lot of test cases.

Comments welcome.

[Araq]

Once we moved io.nim out of system.nim (compatible with our version 2 goals) it should be easier to provide a more elaborate implementation, ideally one that avoids libc and calls into Windows/Posix directly.