Improve error handling

In case of error cluster could get stuck in any state because most parts of sahara code don't expect any errors. State changes must be more consistent.

We should define some internal states for all steps of sahara workflows and correctly handle errors in them and set more human-friendly states for such failures.

Error handling issues list / discussion: https://etherpad.openstack.org/p/Sahara_Error_Handling_Debugging