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/error.rs

@@ -0,0 +1,17 @@
+// Copyright 2025 The Cloud Hypervisor Authors. All rights reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+
+//! Unified error handling for the block crate.
+//!
+//! # Architecture
+//!
+//! ```text
+//! BlockError                 -- single public error type
+//!  |-- BlockErrorKind        -- small, stable, matchable classification
+//!  |-- ErrorContext          -- optional diagnostic metadata (path, offset, op)
+//!  +-- source                -- format-specific error (boxed)
+//!         |-- QcowError
+//!         |-- VhdError / RawError / ...
+//!         +-- io::Error / etc.
+//! ```

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")]