Comment 4 for bug 1399687

We can add target_version_detail just like target_build_number, thus:

If no CheckForUpdate has been called yet, the value is the empty string.

If there is no update available, target_version_detail == version_detail

If there is an update available, target_version_detail will be the version_detail string from the index.json file's Image that the update would leave you on.