Error when logging with a logger that has no writeExtra method

[phil-davis]

Steps to reproduce

1. use some logger that does not implement a writeExtra method
2. have events happen that provide extra fields that could be logged

Expected behaviour

The extra fields are ignored, and the basic log message/data is logged.

Actual behaviour

call_user_func() expects parameter 1 to be a valid callback, class 'OC\Log\Syslog' does not have a method 'writeExtra' at /var/www/html/owncloud/lib/private/Log.php#398

Server configuration

Current GitHub master, or 10.4.1 (or any "recent" release, I expect)