Misleading exception when replica server not found

If the number of Couchbase servers is not enough to provide the required number of replicas (e.g. when installing a stand-alone server and forgetting to uncheck the 'Enable replicas' option), observe throws an ArrayIndexOutOfBoundsException. An exception with a more descriptive name would indicate the error cause more clearly.

An example stack trace:

ArrayIndexOutOfBoundsException: -1
     java.util.ArrayList.elementData(ArrayList.java:371)
     java.util.ArrayList.get(ArrayList.java:384)
     com.couchbase.client.vbucket.config.DefaultConfig.getServer(DefaultConfig.java:81)
     com.couchbase.client.vbucket.VBucketNodeLocator.getServerByIndex(VBucketNodeLocator.java:112)
     com.couchbase.client.CouchbaseClient.observe(CouchbaseClient.java:1621)
     com.couchbase.client.CouchbaseClient.observePoll(CouchbaseClient.java:1750)
     com.couchbase.client.CouchbaseClient.set(CouchbaseClient.java:1199)
     ...