[FIXED JENKINS-10081] fixed the double escaping problem.

changelog.html

@@ -67,6 +67,9 @@ Upcoming changes</a>
   <li class=bug>
     File parameter didn't work correctly with matrix projects
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-10108">issue 10108</a>)
+  <li class=bug>
+    Fixed the double escaping problem in the update center error message
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-10081">issue 10081</a>)
   <li class=rfe>
     Groovy script console is now syntax highlighted.
   <li class=rfe>

core/src/main/resources/hudson/model/UpdateCenter/ConnectionCheckJob/row.jelly

@@ -30,7 +30,7 @@ THE SOFTWARE.
     <td id="prepStatus${size(statuses)}">
       <ul>
         <j:forEach var="s" items="${statuses}">
-          <li>${s}</li>
+          <li><j:out value="${s}"/></li>
         </j:forEach>
       </ul>
     </td>