set logger before registrate to an error handler

This has the potential to fix the white screen of death.

The logger was set after the registration to an error handler. Therefore "PHP Fatal"s like this

```
PHP Fatal error:  Call to a member function debug() on a non-object in lib/private/log/errorhandler.php on line 71
Stack trace:
  1. {main}() 										index.php:0
  2. require_once() 								index.php:26
  3. OC::init() 									lib/base.php:985
  4. spl_autoload_call() 							lib/base.php:524
  5. OC\\Autoloader->load() 						lib/base.php:0
  6. require_once() 								lib/autoloader.php:143
  7. OC\\Log->__construct() 						lib/private/legacy/log.php:16
  8. call_user_func:{lib/private/log.php:34}() 		lib/private/log.php:34
  9. OC_Log_Owncloud::init() 						lib/private/log.php:34
 10. file_exists() 									lib/private/log/owncloud.php:49
 11. OC\\Log\\ErrorHandler->onAll() 				lib/private/log/owncloud.php:49
```

where caused.

This change first sets a log handler and then registers the error handler.

cc @icewind1991 @karlitschek @butonic @PVince81