[SR] Gracefully handle attempt to update pendingSyncWrite
[SR] Gracefully handle attempt to update pendingSyncWrite

Instead of throwing an exception (and closing the client connection)
when a client attempts to modify a pendingSyncWrite, return the
correct status code to the caller.

Change-Id: I9b35ec48d3132452e11644e90a673f70429dddb7
Reviewed-on: http://review.couchbase.org/104647
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine