Improve error handling in libpconsole.
Improve error handling in libpconsole.

If we couldn't create the new pipes, then perhaps some other process has
already created them.  I think it's a security hole to start the agent if
the pipe creation failed.

peters/winpty,rprichard/winpty,fracting/winpty,fracting/winpty,rprichard/winpty,peters/winpty,traff/winpty,fracting/winpty,peters/winpty,traff/winpty,rprichard/winpty,traff/winpty,dscho/winpty,rprichard/winpty,traff/winpty,peters/winpty,dscho/winpty,fracting/winpty,dscho/winpty,dscho/winpty