Relay error in key selection to user.

My SSH agent is only forwarding my DSA key (not my RSA) for whatever reason. This results in a
"no RSA keys" error but its swallowed, leaving me with this message instead:

    $ ./bin/vault steam -k
    No passphrase given; pass `-p` or run `vault -cp`

With this change, this error is shown:

    $ ./bin/vault steam -k
    No usable RSA keys were found