Unwrap CompletionException when dealing with CompletionStage in REST server

[geoand]

• Fixes: quarkus rest error handling for WebApplicationExceptions and CompletionStage does not work anymore #41996

[quarkus-bot]

Status for workflow Quarkus CI

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It should be safe to merge provided you have a look at the other checks in the summary.

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Flaky tests - Develocity

⚙️ JVM Tests - JDK 17

📦 independent-projects/resteasy-reactive/server/vertx

✖ org.jboss.resteasy.reactive.server.vertx.test.sse.SseServerTestCase.shouldNotTryToSendToClosedSink - History

• 1 expectation failed. Response body doesn't match expectation. Expected: "true" Actual: false - java.lang.AssertionError

java.lang.AssertionError: 
1 expectation failed.
Response body doesn't match expectation.
Expected: "true"
  Actual: false

	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)