Correctly avoid unanswerable questions :(

Test plan:
http://localhost:8000/exercises/powers_of_complex_numbers_1.html?debug&problem=0&seed=180
http://localhost:8000/exercises/powers_of_complex_numbers_1.html?seed=193&problem=0&debug

fix #42619; fix #42614; fix #42612; fix #42609; fix #42608; fix #42606;
fix #42598; fix #42562; fix #42561; fix #42554; fix #42532; fix #42530;
fix #42524; fix #42508; fix #42501; fix #42492; fix #42489

Auditors: alpert