Address self-review comments

* Fix up the description of what's done in `unhandled_stop`
* Take @ericniebler's suggestion and make it explicitly UB for the
  `coroutine_handle` returned from `await_suspend` to throw from
  `resume()`.

Author: ispeters

include/stdexec/__detail/__connect_awaitable.hpp

@@ -169,8 +169,7 @@ namespace STDEXEC
       {
         __opstate_.__on_stopped();
         // Returning noop_coroutine here causes the __connect_awaitable
-        // coroutine to never resume past the point where it co_await's
-        // the awaitable.
+        // coroutine to never resume past its initial_suspend point
         return __std::noop_coroutine();
       }
 
@@ -251,7 +250,17 @@ namespace STDEXEC
             {
               static_assert(__std::convertible_to<__suspend_result_t, __std::coroutine_handle<>>);
               auto __resume_target = __awaiter_.await_suspend(__coro_);
-              __resume_target.resume();
+              STDEXEC_TRY
+              {
+                __resume_target.resume();
+              }
+              STDEXEC_CATCH_ALL
+              {
+                STDEXEC_ASSERT(false
+                               && "about to deliberately commit UB in response to a misbehaving "
+                                  "awaitable");
+                __std::unreachable();
+              }
               return;
             }
           }