Clear out the cache xmodule_instance after an exception in XModule init

This guarantees that the ErrorModule will be created, instead of
potentially using a partially instatiated broken module. Fixes
[LMS-1532].

common/lib/xmodule/xmodule/tests/test_error_module.py

@@ -119,7 +119,6 @@ class TestErrorModuleConstruction(unittest.TestCase):
         self.descriptor.xmodule_runtime.error_descriptor_class = ErrorDescriptor
         self.descriptor.xmodule_runtime.xmodule_instance = None
 
-    @unittest.expectedFailure
     def test_broken_module(self):
         """
         Test that when an XModule throws an error during __init__, we
@@ -136,7 +135,6 @@ class TestErrorModuleConstruction(unittest.TestCase):
         with self.assertRaises(TestException):
             module = self.descriptor._xmodule
 
-    @unittest.expectedFailure
     @patch.object(ErrorModule, '__init__', Mock(side_effect=TestException))
     def test_broken_error_module(self):
         """

common/lib/xmodule/xmodule/x_module.py

@@ -746,6 +746,10 @@ class XModuleDescriptor(XModuleMixin, HTMLSnippet, ResourceTemplates, XBlock):
                 )
                 self.xmodule_runtime.xmodule_instance.save()
             except Exception:  # pylint: disable=broad-except
+                # xmodule_instance is set by the XModule.__init__. If we had an error after that,
+                # we need to clean it out so that we can set up the ErrorModule instead
+                self.xmodule_runtime.xmodule_instance = None
+
                 if isinstance(self, self.xmodule_runtime.error_descriptor_class):
                     log.exception('Error creating an ErrorModule from an ErrorDescriptor')
                     raise