1841347 - Migrate `gfx_bindings::error::ErrorSource` to use `nsString` for its message field

Status: NEW

(Core :: Graphics: WebGPU, defect, P3)

Description

[Erich Gubler [:ErichDonGubler]]

The ErrorBuffer type is currently used to carry errors via FFI between dom/webgpu/ipc/WebGPUParent.cpp and gfx/wgpu_bindings/src/server.rs. It uses a fixed-size UTF-8 string buffer, which holds contents populated by the std::error::Error trait on the wgpu_bindings side. This means that errors that are "too long" are truncated! Eek!

Let's move this API to use nsString instead. This will have two benefits:

1. We will no longer need to deal with fixed-size buffers for error messages and, by extension, truncate error messages.
2. Greater consistency with other code around it.