[Plugin: Sidebar Login] Make error messages customizable

I see now that you are following the WP convention of including <strong>ERROR</strong>: at the beginning of your messages. This is probably best practice.

New request: Create a filter. Here’s what I have tried:

foreach ($login_errors->get_error_messages() as $error) {
  $error = apply_filters('sidebar_login_error', $error);
  echo '<div class="login_error">' . $error . "</div>\n";
  break;
}

(Maybe that break should be removed, and an ul of errors be created. I suppose for simple sidebar login it is sufficient to show only the first error.)

Then I add this filter to my functions.php:

function change_login_error_message($message)
{
    return str_replace('<strong>ERROR</strong>: ', '', $message);
}
add_filter('sidebar_login_error', 'change_login_error_message');

A bit hacky to be string replacing, but works since you followed convention!