Bug: says downloads are successful despite errors

[melithine]

I've noticed that in several reported issues, there were errors when downloading content, but they were followed up by a message that suggests it was still successful. I think the biggest offenders of this are the single message / single post downloads. We should ensure that that does not happen; it confuses the user, and worse, if it's not properly detecting the failure, it may store the download status incorrectly in the SQLite database, making it harder to download again in the future without wiping out the entire database and doing a fresh scrape of the account.