Fix error_types in dematerialize sender

[jacksonwelles]

Hiyo, I found what looks like a bug in the dematerialize sender.

The issue is here: #611

To summarize error_types of a sender created by dematerialize is usually an invalid type. For example the following code:

#include <unifex/dematerialize.hpp>
#include <unifex/just.hpp>

#include <unifex/materialize.hpp>
#include <type_traits>
#include <variant>

using namespace unifex;

using Sender = std::decay_t<decltype(dematerialize(materialize(just())))>;
using Errors = typename sender_traits<Sender>::template error_types<std::variant>;
int main() {}

(here on godbolt)
fails to compile and gives the error error: no type named 'type' in 'struct unifex::single_type<>'

When we use std::conditional_t to filter the source sender's value_types into ones that start with set_error, sometimes the false branch type isn't valid.

I think the easiest solution is to add a layer of indirection so that the template argument to tuple isn't evaluated when the condition is false, which I've done here.

After this the compiler complained about a missing ::type so I updated that and switched a ::type to _t for consistency.