Don't unregister failing post-exec callbacks automatically

Instead, print a message regarding the failure, pointing to new
`disable_failing_post_execute` trait for skipping failing callbacks.

When this flag is False (the default), failing callbacks will continue to be called.
When True, behavior is unchanged from previous, where callbacks are only allowed to fail once.

IPython/core/interactiveshell.py

@@ -261,6 +261,9 class InteractiveShell(SingletonConfigurable, Magic):
         deep_reload will still be available as dreload().
         """
     )
+    disable_failing_post_execute = CBool(False, config=True,
+        help="Don't call post-execute functions that have failed in the past."""
+    )
     display_formatter = Instance(DisplayFormatter)
     displayhook_class = Type(DisplayHook)
     display_pub_class = Type(DisplayPublisher)
@@ -2426,17 +2429,22 class InteractiveShell(SingletonConfigurable, Magic):
 
                     # Execute any registered post-execution functions.
                     for func, status in self._post_execute.iteritems():
-                        if not status:
+                        if self.disable_failing_post_execute and not status:
                             continue
                         try:
                             func()
                         except KeyboardInterrupt:
                             print >> io.stderr, "\nKeyboardInterrupt"
                         except Exception:
-                            print >> io.stderr, "Disabling failed post-execution function: %s" % func
-                            self.showtraceback()
-                            # Deactivate failing function
+                            # register as failing:
                             self._post_execute[func] = False
+                            self.showtraceback()
+                            print >> io.stderr, '\n'.join([
+                                "post-execution function %r produced an error." % func,
+                                "If this problem persists, you can disable failing post-exec functions with:",
+                                "",
+                                "    get_ipython().disable_failing_post_execute = True"
+                            ])
 
         if store_history:
             # Write output to the database. Does nothing unless