Translate Error and Warning dialog titles (7.0 lp:1297525)

Fix https://bugs.launchpad.net/openerp-web/+bug/1297525 :
> When an except_osv or except_orm is raised in a non-english environment, the title of the Pop-up window is "OpenERP Warning" and there is no way to change that.

> The same is true for "Access Denied"

Translate error messages in web and server
Add lookup `context` in `self` in translate.py's `_` function