Add new config for changing PHP errors privacy

[matiboux]

A new config should be added to the framework:
It should allow the developer to set the PHP errors public or private.

When the config is set to public, errors should be displayed on the web page.
When the config is set to private, errors should be stored in a error log file.

As the PHP error management in quite complex, the config should support MySQL errors first (for protecting mysql logging informations).