Added exception logger plugin for sabre connector

Whenever an exception occurs in the sabre connector code or code called
by it, it will be logged.

This plugin approach is needed because Sabre already catches exceptions
to return them to the client in the XML response, so they don't appear
logged in the web server log.

This will make it much easier to debug syncing issues.
Merge pull request #6907 from owncloud/webdav-logexceptions

Added exception logger plugin for sabre connector