Notify app developers when their manifest is broken (bug 850830)

https://bugzilla.mozilla.org/show_bug.cgi?id=850830

- There are now 4 manifest fetch attempts (the task is ran once every day)
- Developer is now notified on 3rd fetch attempt that there is a problem (*)
- App enters re-review on the 4th attempt, which is the last
- If fetch was successful, developer is notified if there was a validation error (*)

(*) Unless its app is no longer public or already in the re-review queue.