> the error should be better

Yes, that's what I maeant - sorry I should have phrased better. 
I marked by mistake a couple of f-strings for translation and totally forgot about it for a while... it took me some time to figure out what was going on with this NameError popping out of nowhere.

Curiously, xgettext handles this thing better (...or, far worse actually): it just parses the f-string as a regular one, producing something like
msgid "hello {x}"
msgstr ""
But then of course Python will fail *silently* at runtime, never picking up the translated version of the string...