build,win: bail vcbuild if mklink fails

PR-URL: #27216
Reviewed-By: Richard Lau <[email protected]>

Author: refack

vcbuild.bat

@@ -325,7 +325,10 @@ if errorlevel 1 (
 if "%target%" == "Clean" goto exit
 
 :after-build
+rd %config%
+if errorlevel 1 echo "Old build output exists at 'out\%config%'. Please remove." & exit /B
 if EXIST out\%config% mklink /D %config% out\%config%
+if errorlevel 1 exit /B
 
 :sign
 @rem Skip signing unless the `sign` option was specified.