Comment 5 for bug 589715

I had a few minutes today and created a semi-solution here:

https://code.edge.launchpad.net/~ssd7/byobu/byobu-newwindow-check

It's not truly a solution, but it checks to ensure that the command given in the new window dialog is executable. At the very least this would prevent byobu-config from ever writing a .byobu/windows file that leads to this situation.