Show an error message on instance launch error

Since instance creation happens asynchronously, all that currently
happens on the Horizon side is the state changes from Creating to Error.

This change checks the instance state during the Ajax request on the
backend and returns an Ajax error message popup if the status is
'ERROR'.

Change-Id: I259e27b7b6c73a38098514a701c235feb81af143
Closes-bug: 1236168.
Merge "Show an error message on instance launch error"