Introduced common exception superclass

CloudObjects/SDK/ObjectRetrieverFacade.php

@@ -6,7 +6,7 @@
 
 namespace CloudObjects\SDK;
 
-use Exception;
+use CloudObjects\SDK\Exceptions\SDKException;
 use ML\IRI\IRI;
 use ML\JsonLD\Node;
 
@@ -24,7 +24,7 @@ class ObjectRetrieverFacade {
 
 	private static function getInstance() : ObjectRetriever {
 		if (self::$instance === null)
-			throw new Exception('ObjectRetrieverFacade has not been initialized. Call setObjectRetriever() first.');
+			throw new SDKException('ObjectRetrieverFacade has not been initialized. Call setObjectRetriever() first.');
 
 		return self::$instance;
 	}