Improvements to describe-package buffer.

Improvements to describe-package buffer.

* lisp/help.el (help-map): Bind `C-h P' to describe-package.

* lisp/menu-bar.el (menu-bar-describe-menu): Add describe-package.

* lisp/emacs-lisp/package.el (package-refresh-contents): Catch errors
when downloading archives.
(describe-package-1): Add package commentary.
(package-install-button-action): New function.
(package-menu-mode-map): Bind ? to package-menu-describe-package.
(package-menu-view-commentary): Function removed.
(package-list-packages-internal): Hide the `package' package too.