More file refactoring.
Handle exn:fail

- Handle exn:fail of a module load by restarting with a "blank"
  racket/base REPL.

- Handle exn:fail inside read-eval-print-loop by just calling
  read-eval-print-loop again. Note: I'm a little confused because
  read-eval-print-loop source seems to set a continuation prompt that
  should catch this already. But no, I seem to need the with-handlers
  to catch it.
A few tweaks.
raise & with-handlers => channel-put & break-thread

Don't need to use raise and with-handlers. Instead, sufficient to
channel-put the message. Just also break (or suspend or kill the
thread) so that it doesn't run a bit longer.

Also: lambda -> λ