More useful error when host key verification fails

This changes the error message when host key verification fails from:

```
fatal: [vagrant] => SSH encountered an unknown error during the connection. We recommend you re-run the command using -vvvv, which will enable SSH debugging output to help diagnose the issue
```

to:

```
fatal: [vagrant] => SSH Error: No ECDSA host key is known for [127.0.0.1]:2222 and you have requested strict checking.
Host key verification failed.
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
```

Full output:

```
$ bin/ansible-playbook playbook.yml

PLAY [vagrant] ****************************************************************

GATHERING FACTS ***************************************************************
fatal: [vagrant] => SSH Error: No ECDSA host key is known for [127.0.0.1]:2222 and you have requested strict checking.
Host key verification failed.
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.

TASK: [command uname -a] ******************************************************
FATAL: no hosts matched or all hosts have already failed -- aborting


PLAY RECAP ********************************************************************
           to retry, use: --limit @/Users/marca/playbook.retry

vagrant                    : ok=0    changed=0    unreachable=1    failed=0
```