Richard,
You really should turn error reporting off on live production sites, only enable it on development sites or when actually trying to debug something. Enabling it not only causes "cosmetic" issues, it can break things in unpredictable ways (like causing Javascript errors when error reporting messes up JSON responses in AJAX calls), and it can expose information you don't want people knowing about your site (file paths, dataset naming, etc).
You can safely ignore this specific notice, it doesn't impact the functionality of CMandril at all. If it bugs you, just edit the file as per the PR referenced in my post, or download the fixed file from my fork on github
Hugh