eww: minor code fixes and toolbar support

eww: minor code fixes and toolbar support

* net/eww.el (eww-back-url, eww-forward-url, eww-next-url)
(eww-previous-url, eww-up-url, eww-top-url, eww-add-bookmark)
(eww-bookmark-prepare, eww-bookmark-kill, eww-bookmark-yank)
(eww-bookmark-browse, eww-next-bookmark, eww-previous-bookmark):
Use `user-error'.
(eww-bookmark-mode-map): Add menu.
(eww-render, eww-mode): Use `setq-local'.
(eww-tool-bar-map): New variable.
(eww-mode): Set `tool-bar-map'.
(eww-view-source): Check for `html-mode' with `fboundp'.