Description 
 Use the global $error to get the reason why the login failed. If the username is blank, no error will be set, so assume blank username on that case.
 Plugins extending this function should also provide the global $error and set what the error is, so that those checking the global for why there was a failure can utilize it later.
 See also 
     Parameters 
  - $username
-   (string) (Required) User's username  
- $password
-   (string) (Required) User's password  
- $deprecated
-   (string) (Optional) Not used  Default value: '' 
Return 
 (bool) True on successful check, false on login failure.
    Source 
  File: wp-includes/pluggable-deprecated.php 
 function wp_login($username, $password, $deprecated = '') {
	_deprecated_function( __FUNCTION__, '2.5.0', 'wp_signon()' );
	global $error;
	$user = wp_authenticate($username, $password);
	if ( ! is_wp_error($user) )
		return true;
	$error = $user->get_error_message();
	return false;
}