Proper error handling in dnodeOpenVnodes / dnodeCloseVnodes

* Description
These two functions do not properly handle any error returned from
dnodeGetVnodeList(). This patch als changes the signature of
dnodeGetVnodeList() to make sure it returns only status, and number
of vnodes is passed out through a separate argument.

* Testing
Before the patch, if I start taosd and then remove 'vnode' folder,
the shutdown message would include the following:
05/03 14:26:17.158204 0x140025950476096 DND total vnodes:-2147483296 are
all closed

This shows the code uses error code as vnode count.

After the patch, it outputs proper error message:
05/03 14:30:45.044762 0x139933723604800 DND Get dnode list failed

src/dnode/src/dnodeMgmt.c

@@ -71,13 +71,13 @@ void dnodeMgmt(SRpcMsg *pMsg) {
   rpcFreeCont(pMsg->pCont);
 }
 
-static int32_t dnodeGetVnodeList(int32_t vnodeList[]) {
+static int32_t dnodeGetVnodeList(int32_t vnodeList[], int32_t *numOfVnodes) {
   DIR *dir = opendir(tsVnodeDir);
   if (dir == NULL) {
     return TSDB_CODE_NO_WRITE_ACCESS;
   }
 
-  int32_t numOfVnodes = 0;
+  *numOfVnodes = 0;
   struct dirent *de = NULL;
   while ((de = readdir(dir)) != NULL) {
     if (strcmp(de->d_name, ".") == 0 || strcmp(de->d_name, "..") == 0) continue;
@@ -86,21 +86,28 @@ static int32_t dnodeGetVnodeList(int32_t vnodeList[]) {
       int32_t vnode = atoi(de->d_name + 5);
       if (vnode == 0) continue;
 
-      vnodeList[numOfVnodes] = vnode;
-      numOfVnodes++;
+      vnodeList[*numOfVnodes] = vnode;
+      (*numOfVnodes)++;
     }
   }
   closedir(dir);
 
-  return numOfVnodes;
+  return TSDB_CODE_SUCCESS;
 }
 
 static int32_t dnodeOpenVnodes() {
   char vnodeDir[TSDB_FILENAME_LEN * 3];
   int32_t failed = 0;
-
   int32_t *vnodeList = (int32_t *)malloc(sizeof(int32_t) * TSDB_MAX_VNODES);
-  int32_t  numOfVnodes = dnodeGetVnodeList(vnodeList);
+  int32_t numOfVnodes;
+  int32_t status;
+
+  status = dnodeGetVnodeList(vnodeList, &numOfVnodes);
+
+  if (status != TSDB_CODE_SUCCESS) {
+    dPrint("Get dnode list failed");
+    return status;
+  }
 
   for (int32_t i = 0; i < numOfVnodes; ++i) {
     snprintf(vnodeDir, TSDB_FILENAME_LEN * 3, "%s/vnode%d", tsVnodeDir, vnodeList[i]);
@@ -115,7 +122,15 @@ static int32_t dnodeOpenVnodes() {
 
 static void dnodeCloseVnodes() {
   int32_t *vnodeList = (int32_t *)malloc(sizeof(int32_t) * TSDB_MAX_VNODES);
-  int32_t  numOfVnodes = dnodeGetVnodeList(vnodeList);
+  int32_t numOfVnodes;
+  int32_t status;
+
+  status = dnodeGetVnodeList(vnodeList, &numOfVnodes);
+
+  if (status != TSDB_CODE_SUCCESS) {
+    dPrint("Get dnode list failed");
+    return;
+  }
 
   for (int32_t i = 0; i < numOfVnodes; ++i) {
     vnodeClose(vnodeList[i]);
@@ -143,7 +158,7 @@ static int32_t dnodeProcessCreateVnodeMsg(SRpcMsg *rpcMsg) {
   for (int32_t j = 0; j < pCreate->cfg.replications; ++j) {
     pCreate->nodes[j].nodeId = htonl(pCreate->nodes[j].nodeId);
   }
-  
+
   void *pVnode = vnodeAccquireVnode(pCreate->cfg.vgId);
   if (pVnode != NULL) {
     int32_t code = vnodeAlter(pVnode, pCreate);