[FrameworkBundle] Fixed Translation loader and update translation command.

| 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** :
![capture d ecran 2014-12-20 a 14 55 52](https://cloud.githubusercontent.com/assets/667519/5515053/09f4655c-8859-11e4-8231-fc6587ace5eb.png)

**After, when loading messages** :
![capture d ecran 2014-12-20 a 15 01 36](https://cloud.githubusercontent.com/assets/667519/5515056/216a9fbc-8859-11e4-87a2-9e452df708a5.png)

---------------------------

**Before, when writing messages** :
![capture d ecran 2014-12-20 a 14 57 50](https://cloud.githubusercontent.com/assets/667519/5515058/508c3184-8859-11e4-9456-f8e47190d3cd.png)

**After, when writing messages** :
![capture d ecran 2014-12-20 a 15 03 47](https://cloud.githubusercontent.com/assets/667519/5515062/711f61d2-8859-11e4-8d15-d27700ac0a0f.png)

---------------------------

**Before when loading message and there was no messages found**:
![capture d ecran 2014-12-20 a 15 05 36](https://cloud.githubusercontent.com/assets/667519/5515065/afe422a4-8859-11e4-9d2d-8f8399555462.png)

**After when loading message and there was no messages found**:
![capture d ecran 2014-12-20 a 15 06 48](https://cloud.githubusercontent.com/assets/667519/5515070/da0247dc-8859-11e4-8cd7-127435441ffd.png)

What about moving the condition to the beginning of the method? And returning there is the directory does not exist.