don't blow up rake tasks if an ember app has been deleted

if someone rm -rf's a /app/coffeescript/ember/some_app
on their computer, pushes that to git and then
you check it out, your static asset generation
rake tasks will blow up because files that still
exist but were .gitignor'ed.

this fixes that.


test plan:
* go to a branch that had the 'files' ember app
  (like master from 3 days ago)
* run rake canvas:compile_assets
* now check this out
* run rake canvas:compile_assets again, 
* it should work

Change-Id: Ie777e56b0e09a2f1b236caefb17d10920ddccdcd
Reviewed-on: https://gerrit.instructure.com/38040
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>