Avoid error msg in mapproxy

When building mapproxy part in our respective environnements, we always get this error :

Traceback (most recent call last):
  File "/home/ltgal/mf-chsdi3/mapproxy/createWsgi.py", line 63, in <module>
    os.chown(target, 2002, geodata.gr_gid)

This PR is to avoid printing this error msg.