Suggest running check when we cannot get the volume.

nixops/backends/ec2.py

@@ -507,7 +507,7 @@ def create_after(self, resources, defn):
     def attach_volume(self, device, volume_id):
         volume = nixops.ec2_utils.get_volume_by_id(self.connect(), volume_id)
         if not volume:
-            raise Exception("volume {0} doesn't exist".format(volume_id))
+            raise Exception("volume {0} doesn't exist, run check to update the state of the volume".format(volume_id))
         if volume.status == "in-use" and \
             self.vm_id != volume.attach_data.instance_id and \
             self.depl.logger.confirm("volume ‘{0}’ is in use by instance ‘{1}’, "