Improve the look of torrents with errors a bit (Qt client)

Improve the look of torrents with errors a bit (Qt client)

Display emblem over mime icon if torrent has error(s) to indicate the
issue.
Use normal text color instead of red when drawing selected item: there
is no easy way to be sure that red color looks nice on selection
background, and most of the time it doesn't. Using red color for non-
selected items is questionable too, but let's leave it at that for now.