Always add core templates to twig search paths.

This is because some templates, e.g. errror, expect to be able to
include them. This fixes e.g. the No State error page.

lib/SimpleSAML/XHTML/Template.php

@@ -167,7 +167,7 @@ class Template
         $this->twig_template = ($namespace !== null) ? '@'.$namespace.'/'.$filename : $filename;
         $loader = new TemplateLoader();
         $templateDirs = $this->findThemeTemplateDirs();
-        if ($this->module) {
+        if ($this->module && $this->module != 'core') {
             $templateDirs[] = [$this->module => TemplateLoader::getModuleTemplateDir($this->module)];
         }
         if ($this->theme['module']) {
@@ -180,6 +180,8 @@ class Template
             }
         }
 
+        $templateDirs[] = ['core' => TemplateLoader::getModuleTemplateDir('core')];
+
         // default, themeless templates are checked last
         $templateDirs[] = [
             \Twig_Loader_Filesystem::MAIN_NAMESPACE => $this->configuration->resolvePath('templates')

templates/error.twig

@@ -10,7 +10,9 @@
        e.g. "core:no_state.tpl.php". The format is "<module>:<template name>"
     #}
     {% if includeTemplate -%}
-        {% include(includeTemplate) %}
+        {% set parts = includeTemplate|replace({'.tpl.php':'.twig'})|split(':') %}
+        {% set namespacedTemplate = "@" ~ parts[0] ~ "/" ~ parts[1] %}
+        {% include(namespacedTemplate) %}
     {%- endif %}
 
     <div class="message-box">