GH-1426: Improve handling of Broker data store creation failures

Broker::create_master() and Broker::create_clone() now return
a valid value even when there's a failure to open the backend database
(e.g. SQLite filesystem error).  In that case, the returned value can
still be passed into other data store operations, but they'll fail
immediately with an error.  Broker::is_closed() can now also be used to
determine whether the data store creation calls failed.

src/broker/Manager.cc

@@ -1746,6 +1746,10 @@ bool Manager::CloseStore(const string& name)
 		++i;
 		}
 
+	s->second->have_store = false;
+	s->second->store_pid = {};
+	s->second->proxy = {};
+	s->second->store = {};
 	Unref(s->second);
 	data_stores.erase(s);
 	return true;