Show download errors popup for downloads from Contents tab

[HebaruSan]

Motivation

• While investigating Inconsistency Kraken when installing OuterPlanets #930, I noticed that the validation error for KopernicusTech's old Dropbox link was being printed to the log instead of popping up the failed downloads popup. That turned out to be because I was downloading from the Contents tab, where it's not implemented.
• Furthermore, that same error was showing the text for a DownloadErrorsKraken where it should have been getting to the point of the mismatched hash. Turns out that's because that's what the module downloader throws when Cache.Store throws InvalidModuleKraken.

Changes

• Now if a download from the Contents tab fails, the failed downloads popup will appear.
• Now we re-throw the same exception when Cache.Store fails, so the message will be normal.
  • In case there are any other instances of this left or added in the future, the downloader now detects when a download is directly associated with a DownloadErrorsKraken and extracts the original source exceptions when constructing the new DownloadErrorsKraken representing the whole group of downloads.