cbstats error from non-data nodes. Return friendly error message

cbstats output from non-data nodes return a non-friendly error message like the below.

It should return a friendly message like "cbstats must be executed against a data node"

Traceback (most recent call last):

  File "/opt/couchbase/lib/python/cbstats", line 762, in <module>

    main()

  File "/opt/couchbase/lib/python/cbstats", line 759, in main

    c.execute()

  File "/opt/couchbase/lib/python/clitool.py", line 75, in execute

    f[0](mc, *args[2:], **opts.dict)

  File "/opt/couchbase/lib/python/cbstats", line 37, in g

    f(*args, **kwargs)

  File "/opt/couchbase/lib/python/cli_auth_utils.py", line 75, in g

    mc.bucket_select(bucket)

  File "/opt/couchbase/lib/python/mc_bin_client.py", line 531, in bucket_select

    return self._doCmd(memcacheConstants.CMD_SELECT_BUCKET, name, '')

  File "/opt/couchbase/lib/python/mc_bin_client.py", line 211, in _doCmd

    return self._handleSingleResponse(opaque)

  File "/opt/couchbase/lib/python/mc_bin_client.py", line 204, in _handleSingleResponse

    cmd, opaque, cas, keylen, extralen, data = self._handleKeyedResponse(myopaque)

  File "/opt/couchbase/lib/python/mc_bin_client.py", line 200, in _handleKeyedResponse

    raise MemcachedError(errcode,  msg)

mc_bin_client.ErrorKeyEnoent: Memcached error #1:  KEY_ENOENT : Not Found :