Code review comment for lp:~trapnine/maas/fix-1564971

This looks good in general to me. A few comments below.

I just have one question: I see that you are doing the validation in a signal handler. Normally we would put it in the model or in the form. Was there any particular reason it was done this way? (Is there already a precedent in MAAS code somewhere? I have to admit I haven't done much with signals.)

Did you do any manual testing with the API for this change?