vapp returns false if power action returns bad_request

Fixed issues - where fog models throw exception if it fails to execute
power action on vapp. This happens because the current state of vapp
does not let user execute the action.

e.g If vapp is in power_off state, it throw bad_request if user executes
power_off, suspend,reboot or shutdown action.

In such case, fog model should return false instead of throwing exception. Also
printing the error message.
Merge pull request #2229 from alphagov/handling_bad_request_for_vapp_power_actions

vapp returns false if power action returns bad_request