Verify that sshpass is present for salt-ssh calls

Ref #7528

Salt ssh doesn't appear to tie into the logger at all, so I just used a
print for the error message.  Wasn't sure if logging.getLogger was
correct in this instance.

I'm also flexible on which exception to raise -- SaltClientError seemed
like the most fitting.
Downgrade to a warning, as auth will still work with pubkey
Merge pull request #7549 from basepi/sshpass7528

Verify that sshpass is present for salt-ssh calls