All consts in errors_windows.go (except APPLICATION_ERROR) were
"invented" at the start of windows port to have minimal impact on
existing Go packages. No point keeping them around.
Also remove Errno, since we will be using syscall.Errno everywhere anyway.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/128290044