Title: Fix subpage move error reporting

Title: Fix subpage move error reporting

Title::moveSubpages() sometimes returns a single message-specifier array
and sometimes returns an array of such arrays. This is extremely
difficult for a caller to deal with. Since nothing in Gerrit other than
ApiMove calls this, let's just fix it.

Also, it seems that messages were never created for the errors returned
by this method. So let's create them.

Change-Id: I4e55483c4476a1bb96c87266a4661871776fbf9b