title: Remove unnecessary error message with form validation
body: <p>Using Codeigniter, I'm able to create a function for user authentication.
  I have a function that I call to check if the username/password given are valid.
  The problem I'm having is, I don't want the code to display "Invalid Login" if a password isn't given. It should read "Password is required" or something like that.</p>
  
  <p>I guess I could NOT require the password field, in that case a blank password would result in a failed log in anyways. However, I want to make sure sql injection doesn't occur.</p>
  
  <pre><code>$this-&gt;form_validation-&gt;set_rules('username','username','required|callback_authenticate');
  $this-&gt;form_validation-&gt;set_rules('password','password','required');
  
  $this-&gt;form_validation-&gt;set_message('authenticate','Invalid login. Please try again.');
  </code></pre>
  
  <p>authenticate function</p>
  
  <pre><code>function authenticate() {
  
      return Current_User::checkLogin($this-&gt;input-&gt;post('username'),
                                            $this-&gt;input-&gt;post('password'));
  }
  </code></pre>