Backported logException from stable6
Backported sabre exception logger

Partial backport of 11ef12a

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 #7463 from owncloud/stable5-webdav-logexceptions

[stable5] Backport exception logger for Sabre connector