Move System::httpError to BaseModule->httpError

- This will ensure headers set in BaseModule->run will be carried in httpError scenarios
- Deprecate httpError() method in Core\System

src/Module/Filer/RemoveTag.php

@@ -54,7 +54,7 @@ class RemoveTag extends BaseModule
 
 	protected function post(array $request = [])
 	{
-		System::httpError($this->removeTag($request));
+		$this->httpError($this->removeTag($request));
 	}
 
 	protected function content(array $request = []): string