Expose the Error type for WGSL frontend

[aryaveersr]

Description
This makes the Error type returned by the WGSL frontend public. Previously, it directly returned the error as a string + spans, which is great for cli output but not much else.

Squash or Rebase?

Squash

Checklist

• Run cargo fmt.
• Run taplo format.
  • This seems to modify every .toml file, even ones that I haven't touched. Is this intended ?
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.