Include user.css in error page protostar

[HLeithner]

Pull Request for Issue #25286

Summary of Changes

Load user.css on protostar error page on if needed.

Testing Instructions

create a user.css in the css folder of protostar
trigger a 404 error page

Expected result

user.css gets loaded

Actual result

user.css not loaded

[Quy]

I have tested this item ✅ successfully on 9689130

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[SharkyKZ]

It should be the last CSS file loaded.

[Quy]

I have tested this item ✅ successfully on 62ff495

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[uGE70]

I have tested this item ✅ successfully on 62ff495

user.css is included in error page if there's user.css file in css folder
user.css not included if there's no user.css file.

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[alikon]

RTC

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