Change percent_complete rescue value from 100 to 0
The percent_complete method returns a value of 100 when a ZeroDivisionError occurs. That seems like a very strange default for an error case, and results in a bug when a milestone has no corresponding issues (new, empty milestones show 100% completion). This commit changes the rescue value to 0, and subsequently fixes #1656 (closed), which reported this problem.
