Improve error message in REST Client when no matching readers found

 This would have made my life easier when
 debugging quarkiverse/quarkus-langchain4j#125

(cherry picked from commit 2168a64)

independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/ClientReaderInterceptorContextImpl.java

@@ -91,7 +91,8 @@ public Object proceed() throws IOException, WebApplicationException {
                 }
             }
 
-            StringBuilder errorMessage = new StringBuilder("Response could not be mapped to type " + entityType);
+            StringBuilder errorMessage = new StringBuilder(
+                    "Response could not be mapped to type " + entityType + " for response with media type " + mediaType);
             if (!contextualizers.isEmpty()) {
                 var input = new MissingMessageBodyReaderErrorMessageContextualizer.Input() {
                     @Override