Added MongoCommandException

Changes for 2.13.0 release.

CommandFailureException extends MongoCommandException and is deprecated
UnacknowledgedWriteException extends UnsupportedOperationException instead of MongoClientException and is deprecated

  JAVA-1570
I don't like that the deprecated exception extends the recommended one because it means that you now have to throw the deprecated exception. I would prefer that we always throw the recommended exception instead (using the deprecated one as little as possible).