I get what you are saying, but it really is a theme related issue.
xPaintClass is not the first thing to catch themers out, nor will it be the last.

Themers just need to be vigilant when they release themes.
Also admin verification can be a bonus also (eg; NBI-Studio only archived working themes, but that site is going very slowly)

But the core does need better error messages. I was sure xPC related issues casued an "xPC not found" error... :/