Handle non-AbortError errors in ErrorMiddleware

[MsfPablo]

Refs #3944

Problem

ErrorMiddleware only catches AbortErrors. Any other error thrown from a route escapes the middleware, is converted into a raw 500 by Vapor's internal error handling, and is never logged by the application — so intermittent failures leave no trace.

This also explains the "An unknown error occurred" pages reported in #3944: swift-docc-render shows "page not found" for a 404 on a /data/…json request, but "An unknown error occurred" for any other error status. An unlogged raw 500 from a non-AbortError produces exactly that, with nothing in the logs to diagnose it.

(Credit to @DePasqualeOrg, who tracked this down in #3944 (comment).)

Change

ErrorMiddleware now catches every error, not just AbortError. Non-AbortErrors are treated as .internalServerError, logged at critical with the underlying error description, and rendered with the usual HTML error page. AbortErrors with a status below 400 keep propagating, as before.

The user-facing page only shows the generic "500 - Internal Server Error" text — the underlying error description goes to the log, not the response.

Notes

• This is a visibility/robustness fix; it doesn't itself identify what is intermittently failing, but it makes the failure show up in the logs.
• swift build passes locally. I couldn't run the test suite here (the toolchain in my environment can't resolve the Testing module for the test targets, which also fails on a clean checkout), so the added test is only verified by inspection — please let CI have the final word.

[finestructure]

Thanks for the PR, @MsfPablo ! We have a few high priority PRs that we need to take of at the moment but we'll be looking at this as soon as possible.