fix: deliver projection validation error via exceptional completion

Match how server-side failures of the same condition surface; keep the
public CompletableFuture API free of synchronous throws.

Author: w1am

src/main/java/io/kurrent/dbclient/CreateProjection.java

@@ -30,8 +30,10 @@ public CreateProjection(final GrpcClient client, final String projectionName, fi
     @SuppressWarnings("unchecked")
     public CompletableFuture execute() {
         if (engineVersion == 2 && trackEmittedStreams) {
-            throw new IllegalArgumentException(
-                    "trackEmittedStreams is not supported when engineVersion is 2 (V2)");
+            CompletableFuture<Projectionmanagement.CreateResp> failed = new CompletableFuture<>();
+            failed.completeExceptionally(new IllegalArgumentException(
+                    "trackEmittedStreams is not supported when engineVersion is 2 (V2)"));
+            return failed;
         }
 
         return this.client.run(channel -> {