internal/router: implement PUT for meta endpoints

This also adds the notion of a multi-error, adding a new field to the Error
type.
So in this way we create a slice of nil errors with the given length I presume.
Any reason to not doing this in advance? I guess the reason is that the contract for this function is to always return an empty slice if there is no error. I am ok with that (even if maybe "a slice of error results" is more easy to explain), but I'd clarify that in the BulkIncludeHandler interface decsription.