Move throw of PackInvalidException outside the catch

Move throw of PackInvalidException outside the catch

When a packfile is invalid, throw an exception explicitly
outside any catch scope, so that is not accidentally caught
by the generic catch-all cause, which would set the packfile
as valid again.

Flagging an invalid packfile as valid again would have
dangerous consequences such as the corruption of the in-memory
packlist.

Bug: 544199
Change-Id: If7a3188a68d7985776b509d636d5ddf432bec798
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>