block: Add error module skeleton

Introduce error.rs as the home for a unified error hierarchy that
will replace the per format error types at the public crate
boundary. This commit is intentionally empty beyond the copyright
header and module declaration in lib.rs.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/lib.rs

@@ -9,6 +9,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 pub mod async_io;
+pub mod error;
 pub mod fcntl;
 pub mod fixed_vhd;
 #[cfg(feature = "io_uring")]