Complete user requests exceptionally

* Complete user requests exceptionally if failed to add to the
  processing queue.

Closes jgroups-extras#308.

src/org/jgroups/protocols/raft/RAFT.java

@@ -689,8 +689,9 @@ public CompletableFuture<byte[]> setAsync(byte[] buf, int offset, int length, bo
         }
         if(synchronous) // set only for testing purposes
             handleDownRequest(retval, buf, offset, length, internal, options);
-        else
+        else {
             add(new DownRequest(retval, buf, offset, length, internal, options)); // will call handleDownRequest()
+        }
         return retval; // 4. Return CompletableFuture
     }
 
@@ -705,6 +706,7 @@ protected void add(Request r) {
         }
         catch(InterruptedException ex) {
             log.error("%s: failed adding %s to processing queue: %s", local_addr, r, ex);
+            r.failed(ex);
         }
     }
 
@@ -1298,6 +1300,8 @@ protected void computeMajority() {
 
     protected static class Request {
 
+        protected void failed(Throwable t) { }
+
     }
 
     /** Received by up(Message) or up(MessageBatch) */
@@ -1333,6 +1337,11 @@ public DownRequest(CompletableFuture<byte[]> f, byte[] buf, int offset, int leng
             this.options=opts;
         }
 
+        @Override
+        protected final void failed(Throwable t) {
+            f.completeExceptionally(t);
+        }
+
         public String toString() {
             return String.format("%s %d bytes", DownRequest.class.getSimpleName(), length);
         }