[Grace-core] transitive imports

[James Noble]

> My fault for the typo.  However, my students will surely do worse than that, so we need to make sure the parser doesn't crash on errors.

well yes that would be nice, 
at least we should have the option (perhaps default) of saying that the is an "unspecified syntax error"
and the current input position when the parser crashes.

Michael is on the final stretch of his thesis, so I expect fixing these bugs will be low priority for him now.

but this is the sort of issue we need to keep, to triage into priority lists
and then get sorted well before you start teaching.

in fact, we're looking at having a week of Grace testing at vic from 7 july (between our 1st and 2nd semesters)
ideally this would be caught before then...

J