Fix exception raised for certain non-JSON strings given to is_json

This is breaking pushes in production for certain manifests

Fixes https://jira.prod.coreos.systems/browse/QS-60

util/validation.py

@@ -86,6 +86,6 @@ def is_json(value):
     try:
       json.loads(value)
       return True
-    except TypeError:
+    except (TypeError, ValueError):
       return False
-
+  return False