zig: make JSValue.toBunString use JSError

[nektro]

progress towards #15141

[robobun]

❌ @nektro, your commit f4126ed has 16 failures in #6531:

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

test/integration/next-pages/test/dev-server.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/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-baseline

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

test/js/web/fetch/fetch.tls.test.ts - 1 failing on 🍎 14 aarch64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 aarch64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64-baseline

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64

test/js/node/test/parallel/zlib.test.js - 1 failing on 🐧 20.04 aarch64

test/js/node/child_process/child_process.test.ts - timeout on 🐧 2023 x64

test/js/node/child_process/child_process.test.ts - timeout on 🐧 2023 aarch64

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/js/node/child_process/child_process.test.ts - timeout on 🐧 2023 x64-baseline

test/cli/run/run-crash-handler.test.ts - timeout on 🐧 2023 aarch64

test/cli/run/run-crash-handler.test.ts - timeout on 🐧 2023 x64

test/cli/run/run-crash-handler.test.ts - timeout on 🐧 2023 x64-baseline

test/cli/install/bun-run.test.ts - timeout on 🐧 2023 aarch64

test/cli/install/bun-run.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 11 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 12 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 11 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 20.04 x64

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 🐧 2023 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 2023 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/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 11 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 12 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 20.04 x64-baseline

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 🐧 2023 x64-baseline

test/js/third_party/body-parser/express-bun-build-compile.test.ts - 1 failing on 🐧 2023 x64

test/js/third_party/body-parser/express-bun-build-compile.test.ts - 1 failing on 🐧 2023 aarch64

test/js/third_party/body-parser/express-bun-build-compile.test.ts - 1 failing on 🐧 2023 x64-baseline

test/js/node/test/parallel/timers-ordering.test.js - 1 failing on 🐧 22.04 x64

test/js/node/test/parallel/timers-ordering.test.js - 1 failing on 🐧 22.04 x64-baseline

test/bundler/bundler_compile.test.ts - 1 failing on 🐧 12 aarch64

test/bundler/cli.test.ts - 1 failing on 🐧 2023 aarch64

test/bundler/cli.test.ts - 1 failing on 🐧 2023 x64

test/bundler/cli.test.ts - 1 failing on 🐧 2023 x64-baseline

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

test/cli/install/registry/bun-install-registry.test.ts - timeout on 🐧 22.04 x64-baseline

test/cli/run/require-cache.test.ts - 1 failing on 🪟 2019 x64