Better error message with webpack v4 installed

[bendemboski]

Detect when a too-old version of webpack is installed up front and provide an explicit error message, rather than a more cryptic failure later.

For google-ability, the current error that occurs with webpack v4 installed instead of v5 is:

broccoliBuilderErrorStack: Error: don't know how to insertURL /assets/undefined
    at Placeholder.insertURL (/home/bent/Projekter/Loadbalancer/node_modules/@embroider/webpack/src/html-placeholder.js:43:15)
    at HTMLEntrypoint.render (/home/bent/Projekter/Loadbalancer/node_modules/@embroider/webpack/src/html-entrypoint.js:101:41)
    at Webpack.writeFiles (/home/bent/Projekter/Loadbalancer/node_modules/@embroider/webpack/src/ember-webpack.js:299:105)
  - code: [undefined]
  - codeFrame: don't know how to insertURL /assets/undefined
  - errorMessage: don't know how to insertURL /assets/undefined
        at PackagerRunner (@embroider/webpack)

[ef4]

Thanks!