* lisp/emacs-lisp/package.el: Don't install bad signatures (bug#22089)

* lisp/emacs-lisp/package.el: Don't install bad signatures (bug#22089)

(package--with-response-buffer): NOERROR and ERROR-FORM only
handle connection errors.
(bad-signature): New error type.
(package--check-signature-content): Use it.
(package--check-signature): Properly distinguish connection errors
from bad-signature errors.  Do the check for
`package-check-signature' `allow-unsigned' here instead of forcing
the callbacks to do it.  Add a new argument, UNWIND.
(package--download-one-archive, package-install-from-archive):
Update usage of `package--check-signature'.