jqueue: fix a bug in an error path

Dictionaries raise KeyError, and not ValueError when invalid keys are
passes to del.

Reviewed-by: imsnah

lib/jqueue.py

@@ -499,7 +499,7 @@ class JobQueue(object):
     # Remove master node
     try:
       del self._nodes[self._my_hostname]
-    except ValueError:
+    except KeyError:
       pass
 
     # TODO: Check consistency across nodes