Unify error messages when a filter expects a mapping/sequence

[fabpot]

Besides unifying the error messages, I've also removed the mention of \Traversable as this does not help and is an implementation detail (mapping and sequence are Twig terms, Traversable is not).

Or maybe we should introduce the notion of a "iterable"?

[fabpot]

I would suggest using the concept of "iterable" in those error messages

I've just started to add some docs about Twig types. @drjayvee @ruudk