[perf-web] Fix validation error for missing keys

[Owen Taylor <otaylor src gnome org>]

commit 2d1a3a817f48cb06743bb7a057f445e879fa5baa
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Fri Jul 25 11:01:45 2014 +0200

    Fix validation error for missing keys

 metrics/views.py |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/metrics/views.py b/metrics/views.py
index 4935313..10bc743 100644
--- a/metrics/views.py
+++ b/metrics/views.py
@@ -273,7 +273,7 @@ class ValidationError(Exception):
 def child(parent, key):
     val = parent.get(key, None)
     if val is None:
-        raise ValidationError("'%s' is missing")
+        raise ValidationError("'%s' is missing" % key)
     return val
 
 def child_string(parent, key):