Show what went wrong when sending the email.

Since this is an admin screen, we can show the exception message
unconditionally.

modules/cron/lib/Controller/Cron.php

@@ -162,7 +162,7 @@ class Cron
             $t->data['time'] = $time;
             $t->data['url'] = $url;
             $t->data['mail_required'] = isset($mail);
-            $t->data['mail_sent'] = !isset($e);
+            $t->data['mail_exception'] = $e ?? null;
             $t->data['summary'] = $summary;
             return $t;
         }

modules/cron/templates/croninfo-result.twig

@@ -27,9 +27,11 @@ Tag: {{ tag }}
 </code>
 </div>
 
-{% if mail_required == true and mail_sent == false %}
+{% if mail_required == true and mail_exception %}
 <div class="message-box error">
-Cron-report was not emailed due to an error.
+Cron-report was not emailed due to an error:
+
+<pre>{{ mail_exception.getMessage() }}</pre>
 </div>
 {% endif %}
 {% endblock %}