Check if file was really uploaded.

In addition comment why it does not work.

src/zc/buildout/testing.py

@@ -296,6 +296,8 @@ def make_buildout(executable=None):
 
 def get_filename_from_upload_network_cached(**kw):
   """ Check the upload network cache. """
+  # Note: Since slapos.libnetworkcache is not directly available in testing
+  #       environment this assertions will not work.
   import logging
   import json
   logger = logging.getLogger('zc.buildout')
@@ -304,7 +306,8 @@ def get_filename_from_upload_network_cached(**kw):
                         signature_certificate_file='',
                         signature_private_key_file=None)
   kw.update(default_params)
-  zc.buildout.networkcache.upload_network_cached(**kw)
+  if not zc.buildout.networkcache.upload_network_cached(**kw):
+    return 'Upload failed.'
 
   # check the file created.
   external_url = kw.get('external_url')