Improved error message when entrypoint loading fails

Describe the bug

const EXAMPLE = new MutationObserver();

export default defineContentScript({ ... });

I believe this is the most common question I get asked. Just need a better error message when the error include "Xyz is not defined", and point to this URL: https://wxt.dev/guide/go-further/entrypoint-loaders.html

Reproduction

const EXAMPLE = new MutationObserver();

export default defineContentScript({ ... });

Steps to reproduce

wxt prepare

System Info

System:
    OS: macOS 14.2.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 1.86 GB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.20.3 - ~/.asdf/installs/nodejs/18.20.3/bin/node
    Yarn: 1.22.22 - ~/.asdf/installs/nodejs/18.20.3/bin/yarn
    npm: 10.7.0 - ~/.asdf/plugins/nodejs/shims/npm
    pnpm: 9.12.0 - ~/.asdf/installs/nodejs/18.20.3/bin/pnpm
    bun: 1.1.27 - ~/.bunv/bin/bun
  Browsers:
    Chrome: 129.0.6668.90
    Safari: 17.2.1

Used Package Manager

pnpm

Validations

• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Reproduce the failure with the TypeScript example by running wxt prepare, then trace the entrypoint-loading error described in the issue. The change is done when an undefined-global error produces a clearer message and links to the entrypoint loaders guide.