All handlers now address their own domain of work; :after has become safer;
keyword normalization is multi-stage process; setting use-package-verbose to
`debug' produces useful output in the *use-package* buffer in the case of load
time errors; use-package errors (even internal) won't stop Emacs from
starting (though a serious internal bug that errors out every use-package form
may stop anything from being configured!); and more.
