Better error message when piping no data on "om <path> create --confi…

…g=-"

This scenario produced a 'local variable 'data' referenced before assignment'
unrelevant message.

With patch:

$ echo -n | om c25svc4 create --env port=8003 --config=-
empty feed

Which closes #21

lib/node.py

@@ -2943,11 +2943,14 @@ def svc_conf_from_stdin(self):
         feed = ""
         for line in sys.stdin.readlines():
             feed += line
+        data = None
         if feed:
             try:
                 data = json.loads("".join(feed))
             except ValueError:
                 raise ex.excError("invalid json feed")
+        else:
+            raise ex.excError("empty feed")
         return data
 
     def svc_conf_from_selector(self, selector):