Add test for when_all throwing on individual tasks.

The original behavior for coro::when_all was to capture the T result or
an exception and then when the user iterates all of the completed tasks
the task.return_value() would either return the result or re-throw the
exception for that specific task.

test/test_when_all.cpp

@@ -170,3 +170,38 @@ TEST_CASE("when_all use std::ranges::view", "[when_all]")
     auto result = coro::sync_wait(make_runner_task());
     REQUIRE(result == (1 + 2 + 3));
 }
+
+TEST_CASE("when_all each task throws", "[when_all]")
+{
+    coro::thread_pool tp{};
+
+    auto make_task = [&](uint64_t i) -> coro::task<uint64_t>
+    {
+        co_await tp.schedule();
+        if (i % 2 == 0)
+        {
+            throw std::runtime_error{std::to_string(i)};
+        }
+        co_return i;
+    };
+
+    std::vector<coro::task<uint64_t>> tasks;
+    for (auto i = 1; i <= 4; ++i)
+    {
+        tasks.emplace_back(make_task(i));
+    }
+
+    auto output_tasks = coro::sync_wait(coro::when_all(std::move(tasks)));
+    for (auto i = 1; i <= 4; ++i)
+    {
+        auto& task = output_tasks.at(i - 1);
+        if (i % 2 == 0)
+        {
+            REQUIRE_THROWS(task.return_value());
+        }
+        else
+        {
+            REQUIRE((int)task.return_value() == i);
+        }
+    }
+}