JCBC-490: Error log unexpected/unparsable JSON body content.

Motivation
----------
Stack traces with invalid server responses have been reported where
the root cause is unclear. The stack trace does not show the actual
response however so its hard to decipher.

Modifications
-------------
Since normally such a case shouldn't happen, the actual body including
the HTTP header are logged at ERROR level to raise proper attention and
make it easier to find the actual cause.

Result
------
Unparsable server results that lead to exceptions are now easier to
decipher and to track down eventually.

Change-Id: Id3c98a6488581c5025811433452551de8cc9a136
Reviewed-on: http://review.couchbase.org/39347
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>