Fixed #20278 -- ensured .get() exceptions do not recurse infinitely

A regression caused by d5b93d3281fe93cbef5de84a52 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.
Fixed #20403 -- Ignore forms marked for deletion when validating max_num.
Fixed #20445 -- Raised original exception after command error
Tweaked unit test 'quick start' explanation.

Thanks Jeremy Dunck.
Fixed #20471 - Typos in topics/forms/formsets.txt