3.0 error template

- remove `missing_database.ctp` has `MissingDatabaseException` has been removed
- add a test for `missing_datasource.ctp` to be rendered.

I add to confess that I don't trully understand how the `Error\MissingDatasourceException` is triggered. When I look at the test, the exception seems to be link to `ConnectionManager::get()` but this function throws a `Error\MissingDatasourceConfigException`.
Is this linked to plugin in some way ?