Again, I forgot to make vars.py not crash if the variables aren't set, so we
can print a useful error message. But this time I have the right solution:
vars.py will do the checking for itself, and abort with a nice message.
This doesn't really seem to change anything, but it's more correct and
should reveal weirdness (especially an incorrect .redo directory in a
sub-redo) sooner.
I'm pretty sure this must be the intended behaviour. It's kind of
meaningless to use this to declare a dependency on a file that might start
to exist later, if the file already exists.
