Permanent UnavailableException thrown during servlet request handling should cause servlet destroy about jetty.project HOT 5 CLOSED

@janbartel is this still valid?
Looking at the code I think we got this solved in several different ways.

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@janbartel I think this one is similar to the question on stackoverflow ... https://stackoverflow.com/questions/50907272/get-jetty-to-gracefully-shut-down-a-failed-to-start-webapp-context

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@janbartel this one also overlaps with jakartaee/servlet#152 and our #412

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@joakime stackoverflow and #412/jakartaee/servlet#152 are totally different issues. This issue is purely about how we handle permanent UnavailableExceptions:

• during servlet initialization: the spec says that we should 'release' the servlet ie destroy the instance (but without calling servlet.destroy), but we actually leave it in the ServletHandler, albeit marked as unavailable.

• during request handling: the spec says we should call servlet.destroy and return 404 responses, but we don't call destroy and we return 500 responses instead.

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Fixing this in #4083 with a holder cleanup

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