Prevent error when uploading a new version of an app (bug 927179)

The UI to subscribe to new versions of an app is present but is
not implemented, that's bug 741679. Let's not fail if someones
decides to subscribe in the meantime :-)
Merge pull request #1269 from diox/fix-editorssubscription-marketplace

Prevent error when uploading a new version of an app (bug 927179)