zsh: tweak error state and use bold in prompt
zsh: tweak error state and use bold in prompt

Instead of changing the background on the `#`/`$`, set it on a new
element, `!`, to show error state.

Also, make punctuation bold (`!`, `:`, `#`, `$`).

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent