Add the "exit /b 1" trick to force the batch file to return an error

(to testbuild.js), I suspect the setlocal call is causing the current error
level to not be passed up to the caller. If build.bat failed, testbuild.js
would not always detect the build failure.

src/sbbs3/build.bat

@@ -3,4 +3,4 @@ setlocal
 rem *** Requires Microsoft Visual C++ 2013 ***
 call "%VS120COMNTOOLS%\vsvars32.bat"
 msbuild sbbs3.sln %1 %2 %3 %4 %5
-if errorlevel 1 echo. & echo !ERROR(s) occurred
\ No newline at end of file
+if errorlevel 1 echo. & echo !ERROR(s) occurred & exit /b 1
\ No newline at end of file