emacs-lisp/package.el (package--check-signature): Fix download error handling

emacs-lisp/package.el (package--check-signature): Fix download error handling

* emacs-lisp/package.el (url-recreate-url): Declare.
(url-http-target-url): Declare.
(package-handle-response): Include requested URL in the error
message.
(package--check-signature): Don't re-signal errors from
package--with-work-buffer.  Suggested by Stefan Monnier.