Make `enter!' go into a module even if there was an error when it was instantiated.

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.

collects/racket/enter.rkt

@@ -33,11 +33,15 @@
 (define (do-enter! mod flags)
   (let ([flags (check-flags flags)])
     (if mod
-      (begin (enter-require mod flags)
-             (let ([ns (module->namespace mod)])
-               (current-namespace ns)
-               (unless (memq '#:dont-re-require-enter flags)
-                 (namespace-require 'racket/enter))))
+      (let* ([none "none"]
+             [exn (with-handlers ([void values])
+                    (enter-require mod flags)
+                    none)]
+             [ns (module->namespace mod)])
+        (current-namespace ns)
+        (unless (memq '#:dont-re-require-enter flags)
+          (namespace-require 'racket/enter))
+        (unless (eq? none exn) (raise exn)))
       (current-namespace orig-namespace))))
 
 (struct mod (name timestamp depends))

collects/tests/xrepl/xrepl.rkt

@@ -147,5 +147,11 @@
   ;   located at scribble/html.rkt
   ;   imports: racket/base.rkt, scribble/html/main.rkt.
   ;   no direct exports.
+  -> «(module broken racket/base (define foo 123) (error "bleh!"))»
+  -> «,en broken»
+  bleh!                                 ⇒ threw an error...
+  'broken> «foo»
+  123                                   ⇒ ...but we still got in
+  'broken> «,top»
   -> «,ex»
   @||})