startup_checks: Use fmt::print, fix exec error handling

Uses fmt::print opposed to std::fprintf for error printing.

Call exit instead of returning to caller to prevent a like issue the
previous commit was trying to solve.

Removes unneeded comment.

Co-authored-by: liamwhite <liamwhite@users.noreply.github.com>
Co-authored-by: Lioncash <mathew1800@gmail.com>