title: Custom error message for password presence validation
body: <p>It seems like this should be straightforward. When signing up new users, I want custom errors for blank user names and passwords. It worked fine for the user name:</p>
  
  <pre><code>validates :name, presence: { message: "Please enter a name." },
  
                        length: { maximum: 50, 
                        message: "Please enter a name shorter than 50 characters"}
  </code></pre>
  
  <p>When the field is blank, it gives the "Please enter a name." error.</p>
  
  <p>I the same thing for the password:</p>
  
  <pre><code> has_secure_password
       validates :password, presence: { message: "Please enter a password." }, 
                            length: { minimum: 8,
                               message: "Please choose a password with at least 8 \
                               characters."}
  </code></pre>
  
  <p>The minimum length message works fine. But if I submit with an empty password, I get the default "can't be blank" message.</p>