Extension: Clarify what happens if download is interrupted

In the latest wireframe, the idea now is to force people to download the ISO image through the extension, so the extension should enforce finishing the download before starting the verification. See https://labs.riseup.net/code/attachments/download/764/extension-20150505.fodg.

But we didn’t clarify what happens from a user point of view if the download gets interrupted (lost connection).

Original created by @sajolida on 9717 (Redmine)

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