Steve Lynch

It turns out that (somehow — god only knows how), you can add an external URL as a file attachment in Blackboard. Steve did this and broke things. I think it's now accounted for correctly AND the import receipt is now more informative both here and for missing files.

Also: it looked like we were exiting the file attachment loop early (https://github.com/smtech/canvas/blob/36f2c85f33c021ea108b0b3d1cf352f6d521af6e/www/api/blackboard-import/index.php#L1518) — although, empirically, I don't know if that was really happening. Weird. In any event, we're not breaking there _any more_.
Signed-off-by: Seth Battis <SethBattis@stmarksschool.org>