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

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