Mariadb/Mysql - Break on Check Diff

Check Diff on the `users` task failed cause the `install` task didn't install `python-mysqldb`

```
"The PyMySQL (Python 2.7 and Python 3.X) or MySQL-python (Python 2.X) module is required."
```


Possible solution : 

Use `block` and `rescue` to raise explicitly the error when in check diff + continue running, and return the correct error if failed while not on CD