Help block on form is not meant to display error

I don't see any reasons for putting all `help-block` in form as alert block. @solispauwels

Eventually put this instead in layout.less, I don't know what's that code for:

```css
form.has-error .help-block {
    .alert;
    .alert-danger;
}
```

But for sure not for all `help-block`.