refactor(oxfmt): Prettify external_formatter errors (#19565)

Fixes #19527

### Before

```
Failed to format file with external formatter: /u/p/c/oxc/apps/oxfmt/a.yml
JS formatFile promise rejected: GenericFailure, [object Object]
```

### After

```
SyntaxError: Nested mappings are not allowed in compact mappings (1:6)
[a.yml]
> 1 | key: value: nested
    |      ^^^^^^^^^^^^^
> 2 |
    | ^
```

Author: leaysgur

apps/oxfmt/src-js/cli/worker-proxy.ts

@@ -44,9 +44,25 @@ export async function formatFile(
   options: FormatFileParam["options"],
   code: string,
 ): Promise<string> {
-  return pool!.run({ options, code } satisfies FormatFileParam, {
-    name: "formatFile",
-  });
+  return (
+    pool!
+      .run({ options, code } satisfies FormatFileParam, { name: "formatFile" })
+      // `tinypool` with `runtime: "child_process"` serializes Error as plain objects via IPC.
+      // (e.g. `{ name, message, stack, ... }`)
+      // And napi-rs converts unknown JS values to Rust Error by calling `String()` on them,
+      // which yields `"[object Object]"` for plain objects...
+      // So, this function reconstructs a proper `Error` instance so napi-rs can extract the message.
+      .catch((err) => {
+        if (err instanceof Error) throw err;
+        if (err !== null && typeof err === "object") {
+          const obj = err as { name: string; message: string };
+          const newErr = new Error(obj.message);
+          newErr.name = obj.name;
+          throw newErr;
+        }
+        throw new Error(String(err));
+      })
+  );
 }
 
 export async function sortTailwindClasses(

apps/oxfmt/src/core/external_formatter.rs

@@ -327,9 +327,9 @@ fn wrap_init_external_formatter(
             match status {
                 Ok(promise) => match promise.await {
                     Ok(languages) => Ok(languages),
-                    Err(err) => Err(format!("JS initExternalFormatter promise rejected: {err}")),
+                    Err(err) => Err(err.reason.clone()),
                 },
-                Err(err) => Err(format!("Failed to call JS initExternalFormatter callback: {err}")),
+                Err(err) => Err(err.reason.clone()),
             }
         });
         drop(guard);
@@ -352,9 +352,9 @@ fn wrap_format_embedded(
             match status {
                 Ok(promise) => match promise.await {
                     Ok(formatted_code) => Ok(formatted_code),
-                    Err(err) => Err(format!("JS formatEmbeddedCode promise rejected: {err}")),
+                    Err(err) => Err(err.reason.clone()),
                 },
-                Err(err) => Err(format!("Failed to call JS formatEmbeddedCode callback: {err}")),
+                Err(err) => Err(err.reason.clone()),
             }
         });
         drop(guard);
@@ -377,9 +377,9 @@ fn wrap_format_file(
             match status {
                 Ok(promise) => match promise.await {
                     Ok(formatted_code) => Ok(formatted_code),
-                    Err(err) => Err(format!("JS formatFile promise rejected: {err}")),
+                    Err(err) => Err(err.reason.clone()),
                 },
-                Err(err) => Err(format!("Failed to call JS formatFile callback: {err}")),
+                Err(err) => Err(err.reason.clone()),
             }
         });
         drop(guard);

apps/oxfmt/src/core/format.rs

@@ -219,10 +219,23 @@ impl SourceFormatter {
         }
 
         external_formatter.format_file(external_options, source_text).map_err(|err| {
-            OxcDiagnostic::error(format!(
-                "Failed to format file with external formatter: {}\n{err}",
-                path.display()
-            ))
+            // NOTE: We are trying to make the error from oxc_formatter and external_formatter (Prettier) look similar.
+            // Ideally, we would unify them into `OxcDiagnostic`,
+            // which would eliminate the need for relative path conversion.
+            // However, doing so would require:
+            // - Parsing Prettier's error messages
+            // - Converting span information from UTF-16 to UTF-8
+            // This is a non-trivial amount of work, so for now, just leave this as a best effort.
+            let relative = std::env::current_dir()
+                .ok()
+                .and_then(|cwd| path.strip_prefix(cwd).ok().map(Path::to_path_buf));
+            let display_path = relative.as_deref().unwrap_or(path).to_string_lossy();
+            let message = if let Some((first, rest)) = err.split_once('\n') {
+                format!("{first}\n[{display_path}]\n{rest}")
+            } else {
+                format!("{err}\n[{display_path}]")
+            };
+            OxcDiagnostic::error(message)
         })
     }