`exec::finally()` doesn't include error/stopped signatures from final-sender in completion-signatures

[lewissbaker]

The following code-snippet fails to compile because let_error() computes the variant to hold any potential error produced by finally, but the completion-signatures listed by finally do not include the error-types of the final-sender, despite the finally-algorithm forwarding through these errors.

https://godbolt.org/z/av9rhrbPe

#include <stdexec/execution.hpp>
#include <exec/finally.hpp>

#include <cstdio>

struct X {
    X() : value(0) { std::puts("X::X()"); }
    X(X&& o) noexcept : value(o.value) { std::puts("X::X(X&&)"); }
    X(const X& o) noexcept : value(o.value)  { std::puts("X::X(const X&)"); }
    ~X() { std::puts("X::~X()"); value = -1; }
    int value;
};

int main() {
    auto result = stdexec::sync_wait(
        stdexec::let_error(
            exec::finally(
                stdexec::just(),
                stdexec::just_error(X{})),
            [](const auto& x) noexcept {
                if constexpr (std::same_as<const X&, decltype(x)>) {
                    std::printf("caught X(value=%i)\n", x.value);
                } else {
                    std::printf("caught an exception_ptr\n");
                }
                return stdexec::just();
            }));
}