Change the return values of following functions to return both
error/success and an error/success message:

* pkg_flag()
* pkg_unflag()
* pkg_adopt()
* pkg_vote()
* pkg_delete()
* pkg_notify()
* pkg_delete_comment()
* pkg_change_category()

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>