die-on=warning didn't die on RFC2119 warning

[jyasskin]

https://github.com/w3ctag/user-agents/actions/runs/15033331881/job/42250291624?pr=19 has:

    $ bikeshed --die-on=warning spec "index.bs" "index.bs.built.html" 
      LINE 102: RFC2119 keyword in non-normative section (use: might, can, has to, or override with <span class=allow-2119>): Malicious code on a site must not

But it exited successfully. Even --die-on=everything has an exit code of 0.

[tabatkins]

So the rfc thing is a lint message, not a "warning", so taht's a different category. However, uh, it looks like there's actually no way, right now, to die on a lint message. ^_^

I should probably just switch them to warnings, I'm not sure why I thought they should be something else.