Previously, when trying to `enter!' into a module, an exception would
abort the whole thing, leaving the repl where it was. This behavior can
be very confusing:
> (module foo racket/base (define x 123) (error "bleh!"))
> (enter! 'foo)
bleh!
> x
reference to undefined identifier: x
> (enter! 'foo)
> x
123
Things are a little better with xrepl, since the prompt always indicates
the module that you're in. Still, it's unfortunate that a module with
an error is more likely to be something you'd want to debug -- and most
people are likely to miss the fact that entering a second time will work.
So make `enter!' catch exceptions when requiring the module, change the
namespace, and then re-raise the exception so the user sees it and is
left in the expected namespace.
Set `current-load-relative-directory' to the path of the module file so
paths that it resolves in the description (in `describe-modeul') are not
bogus.
