[WIP] Reorder validations so that unrecoverable errors come first
- Decide how to order the check for basic title validity. Without this, it
makes no sense to check whether the user is allowed to upload to that page.
Currently, $importPlan->getTitle has the side effect of throwing an exception,
which is caught in some unrelated validators and not reported nicely. Maybe
we should keep the explicit check for valid title first?