Don't silently ignore invalid URLs

Scenario: Providing an invalid url via $_GET (wrong/missing extension or file isn't an image (also happens on download errors))
Behavior before: Silently redirect to index
Behavior after: Display error message

core/extension.class.php

@@ -197,6 +197,9 @@ abstract class DataHandlerExtension extends Extension {
 				}
 			}
 		}
+		else{
+			throw new UploadException("Unsupported extension or file isn't an image");
+		}
 	}
 
 	public function onThumbnailGeneration(ThumbnailGenerationEvent $event) {