[tlbuild] Weird "bug" in the build script

[Karl Berry]

    ./Build: exitstatus.txt: cannot create
    cat: cannot open exitstatus.txt
    done (exit status ) Wed Mar 16 18:28:53 CET 2016
    program finished with exit code 0

Ok, I hacked up the last few lines of the Build script to try to detect
exitstatus.txt not containing a number.  Thanks for the report.  -k