Additional errors and checks

It took me about a year of on/off experimenting, searching and annoying the juju devs on IRC to get the Octavia charm to work. I have already opened a separate bug report detailing what I think could be improved and what is missing from the current documentation but another thing that would've helped me greatly in getting octavia to work would have been more and clearer error messages.

Currently, `juju status` lists the octavia unit as being active with a 'unit is ready' message even when there could be several reasons why users with sufficient octavia/OS permissions and resources would fail to create a loadbalancer. `juju status`, when run with the octavia charm installed, should also check that the user has an appropriately tagged amphora image available and it should also check that the user has created a octavia flavor and flavor profile.

I had no idea that Octavia flavors were entirely different from Openstack flavors and a hard requirement of creating OS lbs until I tried to create a LB using the dashboard web interface, when I had already added some normal flavors. I didn't see any error in the octavia logs that gave me any clue that octavia wasn't working because I hadn't created an octavia flavor first.