JVMCBC-95: if error in decodeResponse send to rx

Motivation
----------
Internal errors (exception thrown) in handler's decodeResponse are not
caught until Netty catches them.

Modifications
-------------
Let the user know about these internal errors by calling onError on the
request's Observable, with the internal error wrapped (if not already)
in a CouchbaseException.

Added a test that allows to construct a fake AbstractGenericHandler and
tests this behavior.

Result
------
CouchbaseRequest's observable is now notified via onError if an error
has occurred internally in the handler during response parsing.
All unit tests pass.

Change-Id: I9f5021ea0d9c1f1cc0e34560d5b438e7b48e522d
Reviewed-on: http://review.couchbase.org/44619
Tested-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>