404 is returned if no form has been uploaded.

BUG=https://code.google.com/p/soc/issues/detail?id=2574

Change-Id: I8effc96072117a6afc0ba86cf431213b1c077887
diff --git a/app/soc/modules/gsoc/views/student_forms.py b/app/soc/modules/gsoc/views/student_forms.py
index e03b2d4..2207d44 100644
--- a/app/soc/modules/gsoc/views/student_forms.py
+++ b/app/soc/modules/gsoc/views/student_forms.py
@@ -19,6 +19,8 @@
 from django.utils import html
 from django.utils import translation
 
+from melange.request import exception
+
 from soc.models import document as document_model
 from soc.views.helper import blobstore as bs_helper
 from soc.views.helper import url_patterns
@@ -306,4 +308,8 @@
     else:
       blob_key = self._profile(data).student_data.enrollment_form
 
-    return bs_helper.sendBlobZip(blobstore.BlobInfo(blob_key))
+    if blob_key:
+      return bs_helper.sendBlobZip(blobstore.BlobInfo(blob_key))
+    else:
+      raise exception.NotFound(
+          message='No form has been uploaded for this user.')