Forbid controller and compute servers to be different versions.

Report last compute server error to clients and display in the server summary.

gns3server/controller/gns3vm/__init__.py

@@ -336,6 +336,8 @@ class GNS3VM:
                                 self._controller.notification.emit("log.warning", {"message": msg})
         except ComputeError as e:
             log.warning("Could not check the VM is in the same subnet as the local server: {}".format(e))
+        except aiohttp.web.HTTPConflict as e:
+            log.warning("Could not check the VM is in the same subnet as the local server: {}".format(e.text))
 
     @locked_coroutine
     def _suspend(self):