MDL-50332 enrol_self: Formatting unavailable instances error messages

Many thanks to Andrew Nicols as this patch is based on his
proposal. Thanks Andrew.

enrol/self/classes/empty_form.php

+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Empty enrol_self form.
+ *
+ * Useful to mimic valid enrol instances UI when the enrolment instance is not available.
+ *
+ * @package enrol_self
+ * @copyright 2015 David Monllaó
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+require_once($CFG->libdir.'/formslib.php');
+
+class enrol_self_empty_form extends moodleform {
+
+    /**
+     * Form definition.
+     * @return void
+     */
+    public function definition() {
+        $this->_form->addElement('header', 'selfheader', $this->_customdata->header);
+        $this->_form->addElement('static', 'info', '', $this->_customdata->info);
+    }
+}

enrol/self/lib.php

@@ -234,8 +234,8 @@ class enrol_self_plugin extends enrol_plugin {
 
         $enrolstatus = $this->can_self_enrol($instance);
 
-        // Don't show enrolment instance form, if user can't enrol using it.
         if (true === $enrolstatus) {
+            // This user can self enrol using this instance.
             $form = new enrol_self_enrol_form(NULL, $instance);
             $instanceid = optional_param('instance', 0, PARAM_INT);
             if ($instance->id == $instanceid) {
@@ -243,14 +243,19 @@ class enrol_self_plugin extends enrol_plugin {
                     $this->enrol_self($instance, $data);
                 }
             }
-
-            ob_start();
-            $form->display();
-            $output = ob_get_clean();
-            return $OUTPUT->box($output);
         } else {
-            return $OUTPUT->box($enrolstatus);
-        }
+            // This user can not self enrol using this instance. Using an empty form to keep
+            // the UI consistent with other enrolment plugins that returns a form.
+            $data = new stdClass();
+            $data->header = $this->get_instance_name($instance);
+            $data->info = $enrolstatus;
+            $form = new enrol_self_empty_form(null, $data);
+        }
+
+        ob_start();
+        $form->display();
+        $output = ob_get_clean();
+        return $OUTPUT->box($output);
     }
 
     /**