upload.py: Don't exit if issues with file locking

As seen on a real setup, the /tmp/ didn't provided the proper rights for
the apache user.

As a result, the locking was failing generating an
uncaught raise : upload.py was exiting without warning.

This patch add some traces that explain why the locking wasn't possible.
Upload.py will still fail but at least we know why.

A typical output looks like :

127.0.0.1 - - [14/Jan/2014 14:52:11] "POST //cgi-bin//upload.py
HTTP/1.1" 200 - 'Error on server's lock file : [Errno 13] Permission denied: '/tmp/edeploy.lock''