http: Add log message when work queue is full

More useful error reporting.

src/httpserver.cpp

@@ -286,8 +286,10 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
         assert(workQueue);
         if (workQueue->Enqueue(item.get()))
             item.release(); /* if true, queue took ownership */
-        else
+        else {
+            LogPrintf("WARNING: request rejected because http work queue depth exceeded, it can be increased with the -rpcworkqueue= setting\n");
             item->req->WriteReply(HTTP_INTERNAL, "Work queue depth exceeded");
+        }
     } else {
         hreq->WriteReply(HTTP_NOTFOUND);
     }