flickr and youtube blocks: don't call print_error!

That screws up the entire page the block appears on, when this is just an error local to this block.

blocks/tag_flickr/block_tag_flickr.php

@@ -47,7 +47,10 @@ class block_tag_flickr extends block_base {
         }
 
         if (empty($tagobject)) {
-            print_error('tagnotfound');
+            $this->content = new stdClass;
+            $this->content->text = '';
+            $this->content->footer = '';
+            return $this->content;
         }
 
         //include related tags in the photo query ?

blocks/tag_youtube/block_tag_youtube.php

@@ -86,7 +86,7 @@ class block_tag_youtube extends block_base {
         }
 
         if (empty($tagobject)) {
-            print_error('tagnotfound');
+            return '';
         }
 
         $querytag = urlencode($tagobject->name);
@@ -117,7 +117,7 @@ class block_tag_youtube extends block_base {
         }
 
         if (empty($tagobject)) {
-            print_error('tagnotfound');
+            return '';
         }
 
         $querytag = urlencode($tagobject->name);