Refine error message when making edit to outdated submission version to account for Collect edits

[lognaturel]

Related to #802

Currently, when there's an attempt to edit a deprecated submission version, Central returns a 409 with the following message:

You tried to update a submission, but the copy you were editing (undefined) is now out of date. Please get the new version that has been submitted, and make your edits again.

Enketo does not display the message,

We don't yet know what Web Forms will do with that case.

Collect will soon allow edits to sent forms already on their device. The current message is not appropriate for that. We may want to modify the message or maybe we just let clients design their own messages.