Validations should be revisited

- deprecate sync validations
- callback with error only (reject promise) instead of collecting errors on instance
- think of readable error messages instead of 'Validation error'
- fail after first validation error, when saving
- support `validateAll` method which will perform all validations and collect errors to replace previous functinality of `instance.errors` after `.isValid()`

The issue outlines a major refactor of the validation system in JugglingDB. Start by examining the current validation code, likely in a directory like lib/validations or within model definitions. Look for sync validation methods and error collection logic. Understanding the existing promise/callback patterns and how errors are currently attached to instances is key. The goal is to design a new API that deprecates sync validations, changes error handling, and adds a validateAll method.

Scritto dal modello di indicizzazione a partire dal testo della issue.