Report more information about failures

[jwodder]

• Expand the "x files failed to download" message to distinguish between git-annex addurl failures and metadata command failures.

• Reword the "Downloaded x files" message to emphasize that it counts files fully downloaded & metadata-ed successfully.

  • Alternatively, make the message actually report just the number of successful downloads via git-annex addurl?

• Give Report a summary(&self) -> Summary method that returns a struct containing counts of each type of outcome?