Provide warning if `points_label` is specified, but doesn't match any labels on an issue

[thehale]

I just tried to pull a burndown chart for one of my active projects and got a flat line at 0, but no errors. By debugging the code I realized that my points_label setting was wrong (case sensitive issue - potentially another issue) so none of the point values were getting read.

Better checks here would make for a better user experience.