Move the specific BucketExistsException before the generic CouchBaseException

PR fro Eugene Berger: https://github.com/couchbase/couchbase-net-client/pull/117

Expected Behavior:
When creating a bucket with a name that already exists, the BucketExistsException should be thrown.

Actual Result:
When creating a bucket with a name that already exists, the CouchBaseException is thrown.

How to reproduce:
Create a bucket with the same name twice:
ICluster.IBucketManager.CreateBucketAsync(new BucketSettings{ Name = "NICE_BUCKET"})