Properly log the exception when the endpoint write failed.

Motivation
----------
When the write failed and is logged, the actual exception is not
properly logged too.

Modifications
-------------
Log the cause, which contains the throwable.

Result
------
Better debugable log messages.

Change-Id: I2a60f8cdb5bcf099f7927f7212fe7a0327624985
Reviewed-on: http://review.couchbase.org/44477
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>