Show repo ETag and parsing errors in Failed Downloads

[HebaruSan]

Problems

• If you set up a custom repo of someone's ZIP on a server and then it is later removed, the 404 error will be reported in the message log instead of the Failed Downloads popup:
  
• If you set up a custom repo that points to a file that isn't a ZIP or .tar.gz, the error appears in the message log instead of the Failed Downloads popup:
  

Causes

• If a previously working repo is removed, Net.CurrentETag throws an exception when we try to get the latest ETag
• If a downloaded repo file is in the wrong format, UnsupportedKraken is thrown when RepositoryDataManager.Update calls RepositoryData.FromDownload

The Failed Downloads popup depends on DownloadErrorsKraken, which isn't thrown in either case.

Changes

• Now both classes of errors are redirected into DownloadErrorsKraken exceptions, so they show up in the Failed Downloads popup:
  
  
• The Failed Downloads popup now has horizontal padding so the grid view doesn't abut the edge of the window
• The string reporting the format problem wasn't internationalized and now has been

Fixes #4027.