upload: Adding header on error message

When an error message is sent to the client, it would help the reader to
understand who is sending this message. So let's prefix it with
"upload.py:" to be more explicit

server/upload.py

@@ -329,7 +329,7 @@ def warning_error(error):
 def fatal_error(error):
     '''Report a shell script with the error message and log
     the message on stderr.'''
-    warning_error(error)
+    warning_error("upload.py: " + error)
     sys.exit(1)