See :
http://www.breezejs.com/sites/all/apidocs/classes/ValidationError.html
http://www.breezejs.com/sites/all/apidocs/classes/EntityAspect.html#method_getValidationErrors
Simple example:
var errors = entity.entityAspect.getValidationErrors();
errors.forEach(function(ve) {
var errorMessage = ve.errorMessage;
var property = ve.property;
});
To get all of the errors in an EntityManager you can use
manager.getEntities().forEach(function(entity) {
var errors = entity.entityAspect.getValidationErrors();
//.. do something with the errors ..
});