[FrameworkBundle] Fixed Translation loader.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

This patch verifies that the directory to load translations exists in ``FrameworkBundle/Translation/TranslationLoader.php`` and stop the command if there are no translation found.

**Before, when loading messages** :
- *of a bundle*
![capture d ecran 2014-12-20 a 14 38 05](https://cloud.githubusercontent.com/assets/667519/5514999/e179392a-8855-11e4-8fe1-916bac8c546b.png)

- *of an app folder*
![capture d ecran 2014-12-20 a 14 37 44](https://cloud.githubusercontent.com/assets/667519/5515000/e17bd9a0-8855-11e4-8f55-c32d92039f7c.png)

**After** :
- *of a bundle*
![capture d ecran 2014-12-20 a 14 39 12](https://cloud.githubusercontent.com/assets/667519/5515001/fe65d2aa-8855-11e4-9d5a-2a81bdb2a6e4.png)

- *of an app folder*
![capture d ecran 2014-12-20 a 14 41 34](https://cloud.githubusercontent.com/assets/667519/5515006/63aaceae-8856-11e4-8f0e-1e2fc2cf93c9.png)