Fixes NPE when entity not managed

If one pass a class that is not managed by Achilles, when doing a `sliceQuery(...)` for example, Achilles throws an NPE : 

```
java.lang.NullPointerException
	at info.archinnov.achilles.persistence.AbstractPersistenceManager.sliceQuery(AbstractPersistenceManager.java:167)
	at info.archinnov.achilles.persistence.PersistenceManager.sliceQuery(PersistenceManager.java:467)
```

This PR, makes Achilles throw an `AchillesException` with a better exception message : 

```
info.archinnov.achilles.exception.AchillesException: The entity 'info.archinnov.achilles.test.bugs.NPEWhenClassManaged$NotManaged' is not managed by achilles
```
Shouldn't it be `NPEWhenClass`**Not**`Managed` ?