Raise exception when poll returns error state

When running nova boot --poll, any error states that occur will be
seen but, an exit code of 0 will be returned indicating success.
This would be problematic for scripts looking to see if an
operation completed successfully.

To fix this, an exception is raised after the nova api indicates to the
poll code that the vm is in an error state. The exceptions signals the shell
main function to exit with code 1.

Change-Id: I48f6b1c82e2f8b221dda898dcb804b0196018505
Fixes-Bug: #1292669
Merge "Raise exception when poll returns error state"