Fix menu_pool teardown after exception throwing another exception.

This conceals the original error that you actually want to fix:

	ERROR at teardown of ResourceDetailAppTests.test_left_sidebar_contains_common_metadata
	.ve/local/lib/python2.7/site-packages/django/test/testcases.py:187: in __call__
	    self._post_teardown()
	.ve/src/django-harness/django_harness/override_settings.py:48: in _post_teardown
	    original_post_teardown(innerself)
	.ve/src/django-cms/cms/test_utils/testcases.py:98: in _post_teardown
	    menu_pool.clear()
	.ve/src/django-cms/menus/menu_pool.py:97: in clear
	    cache_keys.delete()
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:467: in delete
	    collector.collect(del_query)
	.ve/local/lib/python2.7/site-packages/django/db/models/deletion.py:166: in collect
	    reverse_dependency=reverse_dependency)
	.ve/local/lib/python2.7/site-packages/django/db/models/deletion.py:77: in add
	    if not objs:
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:100: in __nonzero__
	    self._fetch_all()
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:857: in _fetch_all
	    self._result_cache = list(self.iterator())
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:220: in iterator
	    for row in compiler.results_iter():
	.ve/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py:713: in results_iter
	    for rows in self.execute_sql(MULTI):
	.ve/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py:786: in execute_sql
	    cursor.execute(sql, params)
	.ve/local/lib/python2.7/site-packages/django/db/backends/util.py:47: in execute
	    self.db.validate_no_broken_transaction()
	.ve/local/lib/python2.7/site-packages/django/db/backends/__init__.py:372: in validate_no_broken_transaction
	    "An error occurred in the current transaction. You can't "
	E   TransactionManagementError: An error occurred in the current transaction. You can't execute queries until the end of the 'atomic' block.