Always rebuild the files involved in an error

[bep]

I was testing out some error handling issue an noticed that we, since we're a little conservative about when we rebuild CSS files (PostCSS takes time), and also that the current dependency mechanism have some holes in it (I have a much improved take on this in another branch), it means that when in server mode:

• If you introduce an error in a CSS file, you get the error in the console and in the browser.
• If you fix that error right away, everything is good.
• But you wander on and edit something else that's not connected to the CSS (e.g. a template), the error goes away and does not come back until you edit something CSS related again.

The fix for this would be to always make sure that the files involved in a server error is always in the "change list" until they've been resolved.

[github-actions]

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