fix: add a warning if css urls not exist during build time (fix #9800)

[Megrax]

Description

Closes #9800

Print a warning message if we cannot resolve a css-url during build time.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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• Ideally, include relevant tests that fail without this PR but pass with it.

[Megrax]

@sapphi-red I just added a warning here during build time, due to #9800 (comment). Not sure if this is enough in this situation. 😊

[sapphi-red]

Thanks!

[Megrax]

Hi @sapphi-red @patak-dev, is there any plan to merge this?

[ScopeyNZ]

This warning doesn't consider rollup externals. My project relies on many assets that aren't available to our builder and now our logs are filled with this warning. Should I make a new issue to follow up on this?

[patak-dev]

Yes, thanks!