We'd like to make a much better 404 page for our docs. Basically: incorporate the logo, include the Google site search, and have minimally pleasant visual design. It's this last part that is the problem for the currently active core devs. If you are interested in making a useful contribution to an OSS project, now is your chance! In which case please feel free to chime in on this issue:
Somewhat related it would also be nice to create a simple 403 page for the Bokeh server to display as well.