Racket: fixing help when REPL is in a module

The evaluation of the help form must happen in a good enough
namespace.
Racket: ensuring the first C-c C-d C-i works

... by the obvious device of waiting for the thread building the index
to finish.
Racket: correct detection of manual lookup failures

Still a tad messy, because we are always forcing a retort-syntax error
and checking its ouput, but good enough for now.