application: default to the "stable" branch for update checks

Relates to #372
Relates to #390

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

tpl/page.footer.html

@@ -4,8 +4,16 @@
         {$value}
     {/loop}
 </div>
-{if="$newversion"}
-    <div id="newversion"><span id="version_id">&#x25CF;</span> Shaarli {$newversion} is <a href="https://github.com/shaarli/Shaarli/releases">available</a>.</div>
+{if="$newVersion"}
+<div id="newversion">
+  <span id="version_id">&#x25CF;</span> Shaarli {$newVersion} is
+  <a href="https://github.com/shaarli/Shaarli/releases">available</a>.
+</div>
+{/if}
+{if="$versionError"}
+<div id="newversion">
+  Error: {$versionError}
+</div>
 {/if}
 {if="isLoggedIn()"}
 <script>function confirmDeleteLink() { var agree=confirm("Are you sure you want to delete this link ?"); if (agree) return true ; else return false ; }</script>