Avoid usage of `jl_error()` in `check_cmdline()`
This is the same as https://github.com/JuliaLang/julia/pull/45765 where
we use `jl_error()` too early to get backtraces, but too late to fail
the supposed guard `if` statement that should prevent us from trying to
take a backtrace. X-ref: https://github.com/JuliaLang/julia/issues/45847
