The post can't be saved now unless it has a title and content.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/441).

/cc @bummytime  
I think it belongs in the VC, but it would make more sense to move it to `canSavePost`. If a post has failed, technically it doesn't have changes but we want to allow users to tap 'Save' to retry.