driver: Improve «unhandled ZLE widget 'foo'» error message.
driver: Improve «unhandled ZLE widget 'foo'» error message.

Fixes #409.

x-way/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,stuha/wbr-zsh,danielshahaf/zsh-syntax-highlighting,joshuarubin/dotfiles,danielshahaf/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,joshuarubin/dotfiles,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting