tools: Fix empty password error handling in engine-manage-domains

Fixed empty password error handling in engine-manage-domains (now it
displays clear error message instead of stacktrace).

Change-Id: I7fa948f31694c59ce36fc52cd7eb293a8e0b750f
Bug-Url: https://bugzilla.redhat.com/1083033
Signed-off-by: Martin Perina <mperina@redhat.com>