troubleshooting: add entry for when an exception occurs doing MSET9

_pages/en_US/troubleshooting.txt

@@ -279,6 +279,14 @@ After finishing the MSET9 page, you should switch back to your old SD card befor
 {% endcapture %}
 <details>{{ compat | markdownify }}</details>
 
+{% capture compat %}
+<summary><u>An exception occured after triggering MSET9</u></summary>
+
+This likely indicates that you already have custom firmware. You should [check for CFW](checking-for-cfw).
+
+{% endcapture %}
+<details>{{ compat | markdownify }}</details>
+
 ## Installing boot9strap (super-skaterhax)
 
 {% capture compat %}