Throw better exception if a rule object is missing corresponding Trigger object

If for some reason a rule references a trigger for which `TriggerDB` object doesn't exist, we should explicitly throw this so it's easier to debug.

Before that, cryptic "to_mongo" error was thrown which made it harder to debug and track down the root cause.

```
tomaz@st2master001:~$ st2 rule list
ERROR: 500 Server Error: Internal Server Error
MESSAGE: 'NoneType' object has no attribute 'to_mongo'
```