Submit should fail if file isn't in repo

[Matt Giuca]

It is currently possible to submit a file which does not exist, by hand-editing the submit URL. The submission works all the way until you click "Validate" and only then do you realise the file doesn't exist.

This should do a check at the submit page, and present the user with an error.

[William Grant (wgrant)]

This is difficult, as we cannot really make filesystem requests from the server. Make sure you check at the correct revision.

[Matt Giuca (mgiuca)]

De-milestoning this. It's still a bug, but:
a) It only shows up if you hack the URL,
b) It doesn't "break" anything. It just creates a submission which the lecturer can't check out. So a malicious user can only sabotage their own submission.

So non-critical.