Improved Extension tests and error handling

| Q             | A
| ------------- | ---
| Bug fix?      | yea
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #115 #109
| License       | MIT

- Throw a meaningful exception if a persistence layer has not been
  activated
- Added test for the DI Extension
maybe you saw the discussion here: https://github.com/symfony-cmf/CreateBundle/pull/111 - we might want to do the same in this PR to allow overriding the configuration.