Racket: better help commands

For some reason that i don't fully understand, evaluating a function
in the racket/base namespace first thing after loading errortrace
breaks the help macro (!).  This patches provides a workaround by
actually invoking help first thing when Geiser starts, with alibi that
it serves to preload the help index (in a separate thread).

While i was at it, i improved the message printed in the minibuffer
when no help is found.