Fix scripts/update-contributors.pl CONTRIBUTORS screening (#2125)

Fix scripts/update-contributors.pl CONTRIBUTORS screening (#2125)

    Possible unintended interpolation of @users in string
    Bareword "true" not allowed
    Bareword "false" not allowed
    Execution of ... aborted due to compilation errors

Recent commit daa76f41 broke scripts/update-contributors.pl syntax (see
error messages quoted above) and its lower-case comparison logic,
effectively disabling CONTRIBUTORS checks.

Also do not hide update-contributors.pl execution failures. Buggy
failure detection contributed to the above problems ignored by CI tests.