MapCss stop loading rule on unkonw instruction

[frodrigo]

On the goal of sharing validation between JOSM and Osmose-QA I have implemented MapCSS support into Osmose-QA. Osmose-QA rules may now be written in MapCSS, or converted to MapCSS.
Now I want to move to the next step, share Osmose-QA MapCSS files with JOSM.
On Osmose-QA I have add two "proprietaries" instructions: osmoseItemClassLevel and osmoseTags.
But JOSM fails load rules when it found an unkonw instruction.

2018-11-30 22:06:45.846 INFO: Adding /../home/fred/tmp/osmose-backend/plugins/Bicycle.validator.mapcss to tag checker
2018-11-30 22:06:45.854 SEVERE: Cannot add MapCss rule: Cannot add instruction osmoseItemClassLevel: 4030/40301/2!

Osmose-QA MapCSS rules look like:

way[highway=cycleway][cycleway=track] {
    throwWarning: tr("{0} with {1}", "{0.tag}", "{1.tag}");
    osmoseItemClassLevel: "3032/30328/2";
    osmoseTags: list("tag", "highway", "cycleway");
    fixRemove: "cycleway";
}