When build is failing due to ESLint errors, filter warnings out

Currently, we have a few ESLint rules configured to produce errors. Such as `no-undefined` rule.

However, when we display the error overlay, we *also* show warnings in it. This is confusing because people don’t realize only errors need to be fixed to proceed.

We should fix it so that if the output contains any ESLint errors, we should omit any mention of warnings.