fix Issue 13450 - Add WindowsException and wenforce to std.exception

Also expose ErrnoException's errno field as a documented property
for consistency.
Merge pull request #2505 from CyberShadow/pull-20140909-222639

fix Issue 13450 - Add WindowsException and wenforce to std.exception
Make std.file.chdir safe
Call cenforce in one line
Merge pull request #2558 from tom-tan/safe-file-chdir

Make std.file.chdir safe
Fix issue 13535 byCodeUnit doesn't satisfy hasSlicing