ePub: don't read file that doesn't exist
ePub: don't read file that doesn't exist

fixes CNVS-25458

test plan:
- ePub export regression tests

Change-Id: I87937e47a93e08fbbfd4c23b667ab9384bf63f67
Reviewed-on: https://gerrit.instructure.com/68025
Reviewed-by: Matt Berns <e53c8988cd428dde240324ce2797c8eb7073560f@instructure.com>
Tested-by: Jenkins
Product-Review: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>
QA-Review: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>

grahamb/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,fronteerio/canvas-lms,redconfetti/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,fronteerio/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,redconfetti/canvas-lms,sfu/canvas-lms,redconfetti/canvas-lms,HotChalk/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms