[CX] Use error code for a new error message in CBDataset

In asterix-opt / couchbase-connector/src/main/java/com/couchbase/analytics/metadata/CBDataset.java:

if (bucket.isActive()) \{

  throw new AlgebricksException("Can't drop shadow dataset because its bucket is in the   connected state");

 }

The above error message should use error code.