hooks: handle errors before trying to fetch the output.

vcsserver/hooks.py

@@ -143,8 +143,9 b' def _call_hook(hook_name, extras, writer'
     log.debug('Hooks, using client:%s', hooks_client)
     result = hooks_client(hook_name, extras)
     log.debug('Hooks got result: %s', result)
+
+    _handle_exception(result)
     writer.write(result['output'])
-    _handle_exception(result)
 
     return result['status']