[io] Transform assert() to EMPER_ASSERT_MSG() in GlobalIoContext

We recently saw this assert triggering in our CI. Let's yank up its verbosity.

emper/io/GlobalIoContext.cpp

@@ -36,7 +36,9 @@ static void assert_submitted(ATTR_UNUSED unsigned submitted, ATTR_UNUSED unsigne
 	// Reporting how many sqes were submitted uses khead which
 	// is written asynchronously by the poller kthread
 	if constexpr (emper::IO_SQ_POLLER == IoSqPoller::off) {
-		assert(submitted == expected);
+		EMPER_ASSERT_MSG(submitted == expected,
+										 "Submitted io_uring SQE count "
+												 << submitted << " does not match the expected number of " << expected);
 	}
 }