Fix build.bat/bootstrap.bat exit code (#371)

1. cmd.exe quirk 1: `exit /b 1` sets error code `0` for **the process**, even though it sets `errorlevel` to `1`.
2. `bootstrap.bat` needs to check that `build.bat` succeded (cmd.exe quirk 2: `echo off` doesn't change `errorlevel`).
3. `build.bat` overrides exit code with call to `dir *.exe`.
4. `setlocal` will rollback `pushd` on exit (that also helps with Ctrl+C).

bootstrap.bat

@@ -9,13 +9,15 @@ REM (See accompanying file LICENSE_1_0.txt or http://www.boost.org/LICENSE_1_0.t
 
 :b2_build
 ECHO Building the B2 engine..
+setlocal
 pushd src\engine
 call .\build.bat %*
 @ECHO OFF
-popd
-if exist ".\src\engine\b2.exe" (
+if not errorlevel 1 (
+if exist b2.exe (
+    popd
     copy .\src\engine\b2.exe . > nul
-    goto :b2_built)
+    goto :b2_built))
 goto :b2_failure
 
 
@@ -25,15 +27,11 @@ ECHO Building is done. To install, run:
 ECHO.
 ECHO     .\b2 --prefix=DIR install
 ECHO.
-goto :end
+exit /b 0
 
 
 :b2_failure
 ECHO.
 ECHO Failed to build the B2 engine.
 ECHO.
-goto :end
-
-
-:end
-exit /b %ERRORLEVEL%
+cmd /c exit 1

src/engine/build.bat

@@ -196,7 +196,8 @@ set B2_CXXFLAGS=%B2_CXXFLAGS% -DNDEBUG
 
 @echo ON
 %B2_CXX% %CXXFLAGS% %B2_CXXFLAGS% %B2_SOURCES% %B2_CXX_LINK%
+@if errorlevel 1 goto :Finish
 dir *.exe
 
 :Finish
-@exit /b %ERRORLEVEL%
+@cmd /c exit %ERRORLEVEL%

src/engine/config_toolset.bat

@@ -260,6 +260,6 @@ goto :eof
 :Embed_Minafest_Via_Windres
 if not defined B2_DONT_EMBED_MANIFEST (
     where windres >NUL 2>NUL
-    if %ERRORLEVEL% NEQ 0 ( call; ) else ( set "B2_CXX=windres --input res.rc --output res.o && %B2_CXX% -Wl,res.o" )
+    if errorlevel 1 ( call; ) else ( set "B2_CXX=windres --input res.rc --output res.o && %B2_CXX% -Wl,res.o" )
 )
 goto :eof