homed: return the correct error if an image file is not present when we try to activa...

homed: return the correct error if an image file is not present when we try to activate it

We already return BUS_ERROR_HOME_ABSENT when we detect the image being
absent before we fork off the homework worker. Let's also return the
same error if the homework process notices the same condition while
actually doing something.

This mostly fixes a race, that the same condition seen at different
points in time results in the same errors.