Unify validation error when submitting forms

There are different kinds of validation errors in Studio:

• server side validations, checking consistency, checking potential problems (referenced resources that are missing for instance)
• form validation (when creating a new feature using an id that's already used for instance), where error message is displayed in the form
• poor validation messages, showing an alert

This should be unified to present the error next to the widget allowing to fix the error every time (and this involves defining some kind of layout system to handle validation messages display)