There seem to be some issues with l3messages. I tried to set up some
messages using \err_interrupt_new:NNNnnn. Running this, I first get an
undefined \NamesStart:, which should of course be \NamesStart. A quick
\let:NN then gives me undefined \int_while:nNnT. I think that is fixed
with \let:NN\int_while:nNnT\int_whiledo:nNnT, but my error still looks a
little odd. For example: