@jezek oups, still messaging-app to be built again...
Do you differentiate a download Error with a "Cellular-data off" state ?
For the "data-off", it could be a simple message like on other platforms with a "Download" button.
In the code nuntium returns an error on first connection attempt (there are more stages), but I really don't know if different kind of trouble will cause different error type. I can assume, that if an error happens after first connection attempt, it is not cause of "Celular-data off" state. And I really don't have an idea, how to cause different cause of errors. The plan is to send the error text and/or type to messaging-app via the error message. Maybe then, we will spot the pattern. Meanwhile you can investigate (if you want) and try to cause & get some non "Cellurae-data off" error and then see the log (code producing download errors is here). Maybe, if we can't differentiate by error message, nuntium could ask someone on dbus, or see the config, what the state of cellular switch is and make a note about it in the error message.
Btw, I don't know how are other platforms are handling mms download errors.