Fixes #1146.
We can now write throw Error("file '%s' not found", path); instead of throw Error(format("file '%s' not found") % path); and similarly printError("file '%s' not found", path); instead of printMsg(lvlError, format("file '%s' not found") % path);
This also gets rid of --log-type, since the nested log type isn't useful in a multi-threaded situation, and nobody cares about the "pretty" log type.