[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 mentionned in the bug report
are not handled. 

splbio/openerp-web,MarkusTeufelberger/openerp-web,splbio/openerp-web,MarkusTeufelberger/openerp-web,MarkusTeufelberger/openerp-web,splbio/openerp-web