Fix build.bat/bootstrap.bat exit code

[Kojoley]

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).

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