Improve error reporting when io_uring is unavailable

Ref #55

src/fellow_io_uring.c

@@ -239,6 +239,7 @@ try_flag(unsigned flag)
 {
 	struct io_uring_params params;
 	struct io_uring ring;
+	const char *e;
 	int ret;
 
 	memset(&params, 0, sizeof params);
@@ -250,8 +251,15 @@ try_flag(unsigned flag)
 		return (flag);
 	}
 
-	assert(ret == -EINVAL);
-	return (0);
+	if (ret == -EINVAL)
+		return (0);
+
+	e = strerror(0 - ret);
+	if (e == NULL)
+		e = "NIL";
+	fprintf(stderr, "fellow: io_uring try: FATAL, got %d (%s)\n", ret, e);
+	errno = 0 - ret;
+	WRONG("Unexpected io_uring error. Is it available?");
 }
 
 static void