Record invalid type errors per index with many=True

[tuukkamustonen]

Earlier, errors about invalid types (such as giving primitive for dict) got recorded on _schema level for list elements, rather than for each index separately.

So for example, first block in the added test used to produce:

{
    '_schema': ['Invalid input type.', 'Invalid input type.']}
    0: {},
    1: {}
}

With this change it now produces:

{
    0: {'_schema': ['Invalid input type.']}
    1: {'_schema': ['Invalid input type.']}
}

I don't know marshmallow internals well, and I am quite unsure this. But if this fix makes sense, we can iterate it into a proper form! :)

I used slightly different style to do asserts. I can replace strings by references to Field.default_error_messages, if that's better. Or remove value comparisons altogether. I just think this slightly more verbose form gives reader a better understanding of what is actually in result.errors.

[tuukkamustonen]

Also this, what do you think @sloria? Candidate for 3.0.0?

[tuukkamustonen]

This seems to be fixed (at least in 3.0.0b13).