make eltype throw for some known noniterators

[nsajko]

Neither Nothing or Missing are iterators, iterate(nothing) throws: thus eltype should throw for such arguments as well.

[vtjnash]

I don't believe there is anything about not implementing iteration or getindex which implies that eltype needs to throw or return anything particularly special

[vtjnash]

The PR also breaks a handful of tests, so it is probably not worth considering

[nsajko]

I don't believe there is anything about not implementing iteration or getindex which implies that eltype needs to throw or return anything particularly special

It's better to throw than to return an incorrect result silently. Any possible return value is incorrect, given that a non-iterator doesn't have an element type. Thus throwing seems to be left as the only option.

The PR also breaks a handful of tests, so it is probably not worth considering

I haven't yet looked through all or even most of these, but I'm not disheartened by what I've seen yet. Some of the test failures are benign things, such as @test_throws test that check for a hardcoded error message.