[DX] Improve error when added module is no module

[manniL]

Describe the feature

When adding a npm package to modules that actually is not a Nuxt module (but e.g. exports CSS), the error message is rather cryptic:

e.g. ERROR Cannot start nuxt: expected expression, got ':' 15:56:50

It would be nice to provide a more readable warning and also hint that the module that is failing might not be a module.

Repro

Additional information

• Would you be willing to help implement this feature?
• Could this feature be implemented as a module?

Final checks

• Read the contribution guide.
• Check existing discussions and issues.