these would previously silently fail and simply produce a Nothing if
only some (but not all) of the nested keys were present. This is not
reasonable behaviour that anyone would expect; whenever a nested key
is present, absence of another should be an error (the same goes for
any other errors in that key's definition).

this allows for safer alternative uses for it, such as running it on an
empty list of config sources to just get the list of keys it uses.