[FIX] Test that the config file is readable

This fix should simplify a lot the debugging of broken deployments.

```
[11:32:03] ~ > ~/odoo_src/8.0/openerp-server --addons-path=~/odoo_src/8.0/addons --config .openerp_serverrc
2014-10-22 09:32:20,331 20596 INFO ? openerp: OpenERP version 8.0
2014-10-22 09:32:20,331 20596 INFO ? openerp: addons paths: ['/home/openerp/.local/share/Odoo/addons/8.0', u'/home/openerp/odoo_src/8.0/addons', '/home/openerp/odoo_src/8.0/openerp/addons']
2014-10-22 09:32:20,332 20596 INFO ? openerp: database hostname: localhost
2014-10-22 09:32:20,332 20596 INFO ? openerp: database port: 5432
2014-10-22 09:32:20,332 20596 INFO ? openerp: database user: openerp
2014-10-22 09:32:20,534 20596 INFO ? openerp.service.server: HTTP service (werkzeug) running on 0.0.0.0:8069
^C2014-10-22 09:32:21,571 20596 INFO ? openerp.service.server: Initiating shutdown
2014-10-22 09:32:21,571 20596 INFO ? openerp.service.server: Hit CTRL-C again or send a second signal to force the shutdown.
[11:32:22] ~ > cat .openerp_serverrc
cat: .openerp_serverrc: Permission denied
```