add alert for invalid posenet

Author: pd0wm

car.capnp

@@ -77,6 +77,7 @@ struct CarEvent @0x9b1657f34caf3ad3 {
     sensorDataInvalid @52;
     commIssue @53;
     tooDistracted @54;
+    posenetInvalid @55;
   }
 }