Don't throw if useModuleWorker === false

[eight04]

I use rollup-plugin-iife to convert ES modules into IIFE and use a different loader (i.e. importScripts) to load dependencies. In this case, output.format === 'esm' but I don't want to enable {type: "module"} in the worker.

Here is a live example: https://github.com/eight04/mgcm-skill-data/pull/29/files#diff-6814bf77564b4f1c92f5861e184e28fe217c080a047fefa8b73a728f755ec45c