[gnome-builder] cargo: use NotSupportedError to notify task recipient

[Christian Hergert <chergert src gnome org>]

commit d30aea1c67703cd49ab8cb8638c88eb98cc10342
Author: Christian Hergert <chergert redhat com>
Date:   Sat Oct 29 20:42:48 2016 -0700

    cargo: use NotSupportedError to notify task recipient
    
    They might be checking for NOT_SUPPORTED to determine the difference
    between not supported and failed.

 plugins/cargo/cargo_plugin.py |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/plugins/cargo/cargo_plugin.py b/plugins/cargo/cargo_plugin.py
index b94ce28..73fccc1 100644
--- a/plugins/cargo/cargo_plugin.py
+++ b/plugins/cargo/cargo_plugin.py
@@ -56,7 +56,7 @@ class CargoBuildSystem(Ide.Object, Ide.BuildSystem, Gio.AsyncInitable):
         except Exception as ex:
             task.return_error(ex)
 
-        raise NotImplemented
+        task.return_error(Ide.NotSupportedError())
 
     def do_init_finish(self, task):
         return task.propagate_boolean()