zig: eliminate errorUnionToCPP

[nektro]

cc #15141
the other uses were already deleted in preference of toJSHostValue and toJSHostFunction in earlier patchsets

[robobun]

❌ @nektro, your commit 08b0276 has 14 failures in #6894:

test/regression/issue/09041.test.ts - 1 failing on 🐧 3.20 x64

test/js/bun/http/bun-serve-static.test.ts - timeout on 🐧 3.20 x64-baseline

test/js/node/test/parallel/worker-nested-uncaught.test.js - segmentation fault on 🐧 3.20 x64-baseline

test/js/node/test/parallel/worker-nested-uncaught.test.js - segmentation fault on 🐧 3.20 x64

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64-baseline

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64

test/cli/hot/hot.test.ts - timeout on 🐧 3.20 aarch64

test/js/node/test/parallel/fs-watch-recursive-update-file.test.js - 1 failing on 🍎 14 aarch64

test/js/node/test/parallel/fs-watch-recursive-add-file-with-url.test.js - 1 failing on 🍎 14 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 22.04 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 22.04 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64-baseline

test/integration/next-pages/test/next-build.test.ts - 1 failing on 🐧 22.04 aarch64

test/integration/next-pages/test/next-build.test.ts - 1 failing on 🐧 22.04 x64-baseline

test/integration/next-pages/test/next-build.test.ts - 1 failing on 🐧 12 x64

test/integration/next-pages/test/next-build.test.ts - 1 failing on 🐧 22.04 x64

test/js/bun/spawn/spawn.test.ts - timeout on 🪟 2019 x64

test/cli/install/registry/bun-install-registry.test.ts - 1 failing on 🍎 14 aarch64

[paperdave]

wow i didnt realize the classes generator already got updated. nice