Fail build_task.bat if main nmake failed (GH-17820)

Otherwise we may not notice Windows CI build failures.

Fixes GH-17818.

Author: cmb69

.github/scripts/windows/build_task.bat

@@ -49,6 +49,7 @@ cmd /c configure.bat ^
 if %errorlevel% neq 0 exit /b 3
 
 nmake /NOLOGO
+if %errorlevel% neq 0 exit /b 3
 nmake /NOLOGO comtest.dll
 if %errorlevel% neq 0 exit /b 3