RPM Support

When I run an ISO export task with the --start-date flag, on a repository that hasn't had any package updates between the --start-date specified and the current time, the task fails with a somewhat mysterious message (See pastebin/attachment). I don't think it's necessarily a problem that it's throwing an error, since exporting a null ISO is undesirable. However, if there was a more targeted error message along the lines of "[Errno #] No packages updated within date range supplied", that might help point out the cause of the error.

Attached to this Issue is a copy of the following example:
http://pastebin.com/raw.php?i=pFtNAAMA