fix empty node response

Author: yanhuqing666

src/main/java/com/actiontech/dble/backend/datasource/PhysicalDBPool.java

@@ -506,7 +506,12 @@ public Collection<PhysicalDatasource> getAllDataSources() {
     void getRWBalanceCon(String schema, boolean autocommit, ResponseHandler handler, Object attachment) throws Exception {
         PhysicalDatasource theNode = getRWBalanceNode();
         if (theNode.getConfig().isFake()) {
-            theNode = this.getReadSources().values().iterator().next()[0];
+            if (this.getReadSources().values().size() > 0) {
+                theNode = this.getReadSources().values().iterator().next()[0];
+            } else {
+                String errorMsg = "the dataHost[" + theNode.getHostConfig().getName() + "] is empty, please check it";
+                throw new IOException(errorMsg);
+            }
         }
         if (!theNode.isAlive()) {
             String heartbeatError = "the data source[" + theNode.getConfig().getUrl() + "] can't reached, please check the dataHost";

src/main/java/com/actiontech/dble/server/NonBlockingSession.java

@@ -297,7 +297,7 @@ public void execute(RouteResultset rrs) {
             } catch (Exception e) {
                 handleSpecial(rrs, source.getSchema(), false);
                 LOGGER.info(String.valueOf(source) + rrs, e);
-                source.writeErrMessage(ErrorCode.ERR_HANDLE_DATA, e.toString());
+                source.writeErrMessage(ErrorCode.ERR_HANDLE_DATA, e.getMessage() == null ? e.toString() : e.getMessage());
             }
             if (this.isPrepared()) {
                 this.setPrepared(false);