We've made some adjustments here now. We generally ignore build.json file issues until we're building, where we'll throw an exception at that point if the file exists but doesn't appear to be valid.
For now if you want to edit AddOn.php and change:
PHP:
throw new \LogicException("Unable to read build.json");
To:
PHP:
throw new \LogicException("Unable to read build.json for add-on " . $this->title);
That will at least tell you which is the problematic add-on.