Forgot password with no ID

[catileptic]

Context: In Log in section, forgot your password for an inexistent ID.
Expected behavior: a generic error message that does not give away the fact that an e-mail address or user ID doer or does not exist in the users database.
Actual behavior: message stating “matched several users”, although a random meaningless string was input.
Issue: add an error message when a user ID isn't valid.