return last hub error when unable to create session

Respond to the client with the error message returned
by the last hub tried, when unable to create new session
on any hub. Instead of just responding with the default
error message which does not help a lot.

Author: innokenty

proxy/src/main/java/ru/qatools/gridrouter/RouteServlet.java

@@ -103,6 +103,7 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
         List<Region> unvisitedRegions = new ArrayList<>(allRegions);
 
         int attempt = 0;
+        JsonMessage hubMessage = null;
         try (CloseableHttpClient client = newHttpClient()) {
             while (!allRegions.isEmpty()) {
                 attempt++;
@@ -116,7 +117,7 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 
                     String target = route + request.getRequestURI();
                     HttpResponse hubResponse = client.execute(post(target, message));
-                    JsonMessage hubMessage = JsonMessageFactory.from(hubResponse.getEntity().getContent());
+                    hubMessage = JsonMessageFactory.from(hubResponse.getEntity().getContent());
 
                     if (hubResponse.getStatusLine().getStatusCode() == SC_OK) {
                         String sessionId = hubMessage.getSessionId();
@@ -150,15 +151,23 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
         }
 
         LOGGER.error("[SESSION_NOT_CREATED] [{}] [{}] [{}]", user, remoteHost, browser);
-        replyWithError("Cannot create session on any available node", response);
+        if (hubMessage == null) {
+            replyWithError("Cannot create session on any available node", response);
+        } else {
+            replyWithError(hubMessage, response);
+        }
     }
 
     protected void replyWithOk(JsonMessage message, HttpServletResponse response) throws IOException {
         reply(SC_OK, message, response);
     }
 
     protected void replyWithError(String errorMessage, HttpServletResponse response) throws IOException {
-        reply(SC_INTERNAL_SERVER_ERROR, JsonMessageFactory.error(13, errorMessage), response);
+        replyWithError(JsonMessageFactory.error(13, errorMessage), response);
+    }
+
+    protected void replyWithError(JsonMessage message, HttpServletResponse response) throws IOException {
+        reply(SC_INTERNAL_SERVER_ERROR, message, response);
     }
 
     protected void reply(int code, JsonMessage message, HttpServletResponse response) throws IOException {