Improve the error message when winpty.exe can't start its child process
Improve the error message when winpty.exe can't start its child process

Fixes https://github.com/rprichard/winpty/issues/76.

rprichard/winpty,rprichard/winpty,fracting/winpty,rprichard/winpty,rprichard/winpty,fracting/winpty,fracting/winpty,fracting/winpty