That isn't a valid haiku, and it's much less clear, in my opinion. If you give me a nice big "404 Not Found" up front and then include a poem, HTTP cat, or easter egg, that would give the page personality while still being immediately useful.
Of course, it is still both a 404 HTTP response and a proper HTML webpage - so both the user agent and the website have an opportunity to explain the error. If both of them try to be funny, though, that's no good.
