Add DEBUG print status

[velp]

I could not reproduce the issue in QA, so I added more error handlers and debug messages to catch the situation.
This PR adds:

• additional error handler for empty response
• additional error handler for response less than 2 devices
• more information if the status request failed
• additional check if both devices are BACKUP
• if both devices are not active we do not update the status in the cache and keep the current status

Related to octavia-issues#28

[m-kratochvil]

One test is failing but beside that the change looks ok, approved