[core] extract BridgeAbstract methods to make them functions

- returnError, returnServerError, returnClientError ,debugMessage are
  moved to lib/error.php

- getContents, getSimpleHTMLDOM, getSimpleHTMLDOMCached are moved to
  lib/contents.php

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

bridges/NeuviemeArtBridge.php

@@ -17,7 +17,7 @@ class NeuviemeArtBridge extends FeedExpander {
 	protected function parseItem($item){
 		$item = parent::parseItem($item);
 
-		$article_html = $this->getSimpleHTMLDOMCached($item['uri']);
+		$article_html = getSimpleHTMLDOMCached($item['uri']);
 		if(!$article_html){
 			$item['content'] = 'Could not request 9eme Art: '.$item['uri'];
 			return $item;