Add troubleshooting notes attributing the error page

So that we can tell which error page rendered

CHANGELOG.md

 # Root context and error valve change log
 
+## Next
+
++ Add text attributing the 500 error page.
+
 ## 1.2.1
 
 2022-03-14

myuw-root-webapp/src/main/webapp/500.html

@@ -129,6 +129,10 @@
                   <li role="menuitem"><a href="https://www.wisconsin.edu/ohrwd/benefits/">UW System Employee Benefits Website</a></li>
                 </ul>
               </div>
+
+              <div>
+                <p>Troubleshooting notes: (root) /500.html</p>
+              </div>
             </div>
           </div>
           <div class="mdl-cell mdl-cell--2-col mdl-cell--1-col-phone"></div>