ShellPkg: Print error message when Shell set environment variable fail.

If you try to 'set' a read only environment variable and it fails without printing any information.
This patch add error message printing when 'set' environment variable fails.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18598 6f19259b-4bc3-4df7-8a09-765794883524

ShellPkg/Library/UefiShellLevel2CommandsLib/Set.c

@@ -2,7 +2,7 @@
   Main file for attrib shell level 2 function.
 
   (C) Copyright 2015 Hewlett-Packard Development Company, L.P.<BR>
-  Copyright (c) 2009 - 2010, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2009 - 2015, Intel Corporation. All rights reserved.<BR>
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
   which accompanies this distribution.  The full text of the license may be found at
@@ -141,6 +141,11 @@ ShellCommandRunSet (
         // assigning one
         //
         Status = ShellSetEnvironmentVariable(KeyName, Value, ShellCommandLineGetFlag(Package, L"-v"));
+        if (EFI_ERROR(Status)) {
+          ShellPrintHiiEx(-1, -1, NULL, STRING_TOKEN (STR_SET_ERROR_SET), gShellLevel2HiiHandle, L"set", KeyName);
+          ShellStatus = (SHELL_STATUS) (Status & (~MAX_BIT));
+        }
+        
       } else {
         if (KeyName != NULL) {
           //