handle missing media_id for flvs in epub export

fixes CNVS-25933

test plan:
- Add an flv to a course and include is as downloadable content for a
  module that is accessible to the user.
- Add a media comment to the body of an assignment (or any content
  type).
- Export the course (module sorting), logged in as a user that has
  rights to see the module & the content featuring the media comment.
- Observe that the ePub export is successful.
- Observe that the flv is included in the associated files zip file.
- Observe that the media comment is visible inline in the exported
  ePub.


Change-Id: I796a20da4380ee759ffb1e4cde194ba65a7e599f
Reviewed-on: https://gerrit.instructure.com/69021
Reviewed-by: Matt Berns <mberns@instructure.com>
Tested-by: Jenkins
QA-Review: Deepeeca Soundarrajan <dsoundarrajan@instructure.com>
Product-Review: John Corrigan <jcorrigan@instructure.com>

lib/cc/exporter/epub/converters/files_converter.rb

@@ -10,7 +10,13 @@ module CC::Exporter::Epub::Converters
       attr_reader :flv_path
 
       def convert!
-        return flv_path unless mp4_url.present?
+        begin
+          return flv_path unless mp4_url.present?
+        # Maybe a weird exception to catch, but that's what ends up being thrown
+        # if we pass a media_id that doesn't have a corresponding set of assets.
+        rescue NoMethodError => e
+          return flv_path
+        end
 
         f = File.open(mp4_path, 'wb')
         CanvasHttp.get(mp4_url) do |response|