When reporting compilation errors, include warning strings.

This allows plugins to call mju_warning, and have the string propagate to users.

PiperOrigin-RevId: 650621601
Change-Id: I608e7a3a1ad126ab8da2ff0634fa708458fb9ccc

plugin/actuator/pid.cc

@@ -103,7 +103,7 @@ std::unique_ptr<Pid> Pid::Create(const mjModel* m, int instance) {
   }
 
   if (config.slew_max.value_or(0.0) < 0) {
-    mju_warning("maxslew must be non-negative");
+    mju_warning("slewmax must be non-negative");
     return nullptr;
   }