Don't require importing paramiko for error.

The _run_ssh function raises a paramiko exception, which requires
functions that use it to import the paramiko class. This should be
hidden away, and so I created a new exception in its place. Nobody
but the Storwize/SVC driver use it anyway, and I am updating that
driver in a separate patch.

Change-Id: Ibf353edc664765897a5fa7a08d126de4b8992863
Merge "Don't require importing paramiko for error."