Set the second arg to __init__ for the HeartbeatFailure exception to 'mongo', create a split_mongo connection test

Fixes TNL-2267

Clean up and PEP8 fixes to the test

Add missing paramater back

Add pylint pragma to ignore unused argument warning

common/lib/xmodule/xmodule/modulestore/split_mongo/mongo_connection.py

@@ -309,7 +309,7 @@ class MongoConnection(object):
         if self.database.connection.alive():
             return True
         else:
-            raise HeartbeatFailure("Can't connect to {}".format(self.database.name))
+            raise HeartbeatFailure("Can't connect to {}".format(self.database.name), 'mongo')
 
     def get_structure(self, key, course_context=None):
         """

common/lib/xmodule/xmodule/modulestore/tests/test_split_mongo_mongo_connection.py

@@ -0,0 +1,19 @@
+""" Test the behavior of split_mongo/MongoConnection """
+import unittest
+from mock import patch
+from xmodule.modulestore.split_mongo.mongo_connection import MongoConnection
+from xmodule.exceptions import HeartbeatFailure
+
+
+class TestHeartbeatFailureException(unittest.TestCase):
+    """ Test that a heartbeat failure is thrown at the appropriate times """
+    @patch('pymongo.MongoClient')
+    @patch('pymongo.database.Database')
+    def test_heartbeat_raises_exception_when_connection_alive_is_false(self, *calls):
+        # pylint: disable=W0613
+        with patch('mongodb_proxy.MongoProxy') as mock_proxy:
+            mock_proxy.return_value.alive.return_value = False
+            useless_conn = MongoConnection('useless', 'useless', 'useless')
+
+            with self.assertRaises(HeartbeatFailure):
+                useless_conn.heartbeat()