PYCBC-341: Check for error at last stats callback
PYCBC-341: Check for error at last stats callback

If the stats command failed because of a network error or similar, the
SDK would not catch it or propagate it.

Change-Id: I8dc4e1e1689bfa8e4b179850ce2e9933d65cc4cd
Reviewed-on: http://review.couchbase.org/62576
Reviewed-by: Volker Mische <fb414f8ac0dbbf87663550ae4ef5fc95b1041941@gmail.com>
Reviewed-by: Will Gardner <cbd6f5d9092d9ef7d5c0510d53bba134b22c02f2@couchbase.com>
Tested-by: Mark Nunberg <dab05a0fe859c5cb93882b2a75a0d613501b07be@couchbase.com>

couchbase/couchbase-python-client,couchbase/couchbase-python-client