export.php: wrong error for AggregateMessageGroups

The check for FileBasedMessageGroup was always triggered before
the check for ::isMeta(), so the latter check never matched.
Fixed by changing the order of the checks. Also fixed error
message wording to be correct after refactorings.

Change-Id: I7db87596b54705ef0bc61b7a6794a347407ea77d
Merge "export.php: wrong error for AggregateMessageGroups"