Allow to add errors before validate

[emaglio]

In this is possible to have:

form.errors.add(:email, "already taken)
form.validate(email: "myemail@email.com")
form.errors.messages(email: ["already taken", "error from validation"])

or (maybe more possible):

form.validate(email: "my@email.com")
form.errors.add(:email, "something")
unless form.valid?
  flash[:error] = form.errors.messages
end

An add error will never be clened up by validate.... validate a new form instance to have a fresh start