No error message for the invalid credential for the ssh plugin

if wrong private key is supplied, it just keeps on trying and prints message as `Fetching IP for machine`, there is no clue what is happening. Need further discussion to understand how we can enhance the error message.


different values for following fields:
```
            "key_pair_name": "mkumatag-pub-key",
            "ssh_private_key_file": "/Users/manjunath/.ssh/id_rsa",
```