[FIX] race condition in session directory creation
[FIX] race condition in session directory creation

try to create the directory and handle the possible exception instead of doing
an unsafe 2 step check and creation.

The issues related to the naming of the directory mentioned in the bug report
are not handled.

Fixes lp:1157102


akretion/openerp-web,akretion/openerp-web,akretion/openerp-web