Fix "Error displaying the error page: Application Instantiation Error"

`Error displaying the error page: Application Instantiation Error`

This is IMHO one of the most annoying bugs since... I believe 1.6.
The only reason I know is a missing database connection.

This PR tries to solve this by
* Adding a "lowest exception handler" to the site/index.php
* Removing the exception handling from JErrorPage::render() and deleting this silly messsage...

To test this, shut down your db server or invalidate credentials.

Expected result:
`Could not connect to MySQL.`