Fix headerscheck and cpluspluscheck's exit codes.
For the benefit of CI, which started running these header check scripts
in its CompilerWarnings task in commit
81b9f23c9c8, they should report
failure if any individual header failed to compile.
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGKtDwPo9wzKgbStDwfOhEpywMc6PQofio8fAHR7yUjgxw%40mail.gmail.com