[stable5] Backport exception logger for Sabre connector

To improve sync issue debugging on stable5.

Backport of https://github.com/owncloud/core/pull/6907

Note: I had to copy the method `OCP\Util::logException` from master because it didn't exist on stable5. It is currently only used by the sabre exception logger.

I've tested sync upload + sync download and also tried to trigger an error by adding a "throw" in `connector/file.php` to simulate an error. The exception was logged properly.

Please review @karlitschek @DeepDiver1975 @tanghus