small changes

new cell defaults
new end-of-list index -1
Implemented error-checking and displaying

checks that field have the right types of values in them and alerts the
user if they do not
got edited cells to hightlight with red border

they do it even when there is no diagram drawn yet though, which is not
really desirable behaviour (I think?).  Anyhow, that will be revisited
over the break.