Show error message if history structure can't be shown

Fixes:
```
AttributeError: 'NoneType' object has no attribute 'id'
  File "galaxy/web/framework/middleware/sentry.py", line 43, in __call__
    iterable = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python2.7/site-packages/paste/recursive.py", line 85, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/middleware/statsd.py", line 35, in __call__
    req = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python2.7/site-packages/paste/httpexceptions.py", line 640, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/base.py", line 136, in __call__
    return self.handle_request(environ, start_response)
  File "galaxy/web/framework/base.py", line 215, in handle_request
    body = method(trans, **kwargs)
  File "galaxy/webapps/galaxy/controllers/history.py", line 494, in display_structured
    assert (history.user and (history.user.id == trans.user.id) or
```

lib/galaxy/webapps/galaxy/controllers/history.py

@@ -493,11 +493,10 @@ def display_structured(self, trans, id=None):
         history = trans.sa_session.query(model.History).options(
             eagerload_all('active_datasets.creating_job_associations.job.workflow_invocation_step.workflow_invocation.workflow'),
         ).get(id)
-        assert history
-        # TODO: formalize to trans.show_error
-        assert (history.user and (history.user.id == trans.user.id) or
-                (history.id == trans.history.id) or
-                (trans.user_is_admin()))
+        if not (history and ((history.user and trans.user and history.user.id == trans.user.id) or
+                             (trans.history and history.id == trans.history.id) or
+                             trans.user_is_admin())):
+            return trans.show_error_message("Cannot display history structure.")
         # Resolve jobs and workflow invocations for the datasets in the history
         # items is filled with items (hdas, jobs, or workflows) that go at the
         # top level