issue #1467 avoid error when Piwigo version is not available on PEM

* do not display an error message "cannot connect to server" but simply "no plugin available"
* if we are on a beta/RC version, display a button to switch to test plugins compatible with Piwigo previous version
* add a tooltip with more details about the "show beta test plugins"

admin/include/plugins.class.php

@@ -447,7 +447,7 @@ DELETE FROM '. PLUGINS_TABLE .'
     $versions_to_check = $this->get_versions_to_check($beta_test);
     if (empty($versions_to_check))
     {
-      return false;
+      return true;
     }
 
     // Plugins to check

admin/plugins_new.php

@@ -181,5 +181,10 @@ else
   $page['errors'][] = l10n('Can\'t connect to server.');
 }
 
+if (!$beta_test and preg_match('/(beta|RC)/', PHPWG_VERSION))
+{
+  $template->assign('BETA_URL', $base_url.'&beta-test=true');
+}
+
 $template->assign_var_from_handle('ADMIN_CONTENT', 'plugins');
 ?>

admin/themes/default/template/plugins_new.tpl

@@ -32,7 +32,7 @@ const str_from_begining = "{"since the beginning"|@translate}";
 <div class="titrePage">
   <div class="sort">
     <div class="sort-actions">
-      <div class="beta-test-plugin-switch">
+      <div class="beta-test-plugin-switch tiptip" title="{'Show plugins compatible with previous version of Piwigo'|translate|escape:html}">
         <label class="switch">
           <input type="checkbox" id="showBetaTestPlugin" {if $BETA_TEST}checked{/if}>
           <span class="slider round"></span>
@@ -208,5 +208,9 @@ const str_from_begining = "{"since the beginning"|@translate}";
 {/foreach}
 </div>
 {else}
-<p>{'There is no other plugin available.'|@translate}</p>
+<p>{'There is no other plugin available.'|@translate}
+{if isset($BETA_URL)}
+  <a href="{$BETA_URL}" class="buttonLike icon-fire">{'Show beta test plugins'|@translate}</a>
+{/if}
+</p>
 {/if}