Add an entry in the changelog.

CHANGELOG.md

@@ -113,6 +113,7 @@ the same every time it is rendered, we now warn if it is missing.
 - Validate the number of color attachments in `create_render_pipeline` by @nical in [#2913](https://github.com/gfx-rs/wgpu/pull/2913)
 - Validate against the maximum binding index in `create_bind_group_layout` by @nical in [#2892](https://github.com/gfx-rs/wgpu/pull/2892)
 - Validate that map_async's range is not negative by @nical in [#2938](https://github.com/gfx-rs/wgpu/pull/2938)
+- Expose the cause of the error in the `map_async` callback in [#2939](https://github.com/gfx-rs/wgpu/pull/2939)
 
 #### DX12
 - `DownlevelCapabilities::default()` now returns the `ANISOTROPIC_FILTERING` flag set to true so DX12 lists `ANISOTROPIC_FILTERING` as true again by @cwfitzgerald in [#2851](https://github.com/gfx-rs/wgpu/pull/2851)