JCLOUDS-375: Fix VirtualGuestToNodeMetadata when server returns SoftLayer_Exception_Order_Item_Duplicate

* Wrap getOrderTemplate in VirtualGuestToNodeMetadata.getImage with try/catch
* Address comments from @nacx and @demobox:
  - remove the fields of the custom SoftLayerOrderItemDuplicateException and propagate the HttpResponseException
  - use of order = null instead of return null
  - extend RuntimeException instead IllegalStateException in SoftLayerOrderItemDuplicateException

Conflicts solved:
	providers/softlayer/src/main/java/org/jclouds/softlayer/compute/functions/VirtualGuestToNodeMetadata.java