Better spawn error message (#1434)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Structure preview errors with source-aware messaging/UI and enhance
local/Docker spawn error diagnostics and logging.
> 
> - **Frontend**:
> - **Error model**: Change `previewErrorMessageAtom` from `string` to
`{ message, source }` to distinguish `preview-app` vs `dyad-app` errors.
> - **Preview UI**: Update `ErrorBanner` in
`components/preview_panel/PreviewIframe.tsx` to use `error.message`,
show an "Internal Dyad error" chip for `dyad-app`, conditional tip text,
and hide AI fix for non-`preview-app` errors; use `cn` helper.
> - **Error propagation**: Wrap iframe and build errors via
`setErrorMessage({ message, source: "preview-app" })`; adjust AI prompt
to use `errorMessage.message`.
> - **Hooks**:
> - `useRunApp`: On run/stop/restart failures, set `{ message, source:
"dyad-app" }` in `previewErrorMessageAtom`.
> - **Backend**:
> - `ipc/handlers/app_handlers.ts`: Improve spawn failure handling for
local node and Docker: capture stderr as strings, collect error details
(`message`, `code`, `errno`, `syscall`, `path`, `spawnargs`), log with
context, and throw enriched error messages.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
4135b04e19431dd53848c3266e5211e4c9df6aa2. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

src/ipc/handlers/app_handlers.ts

@@ -152,13 +152,44 @@ async function executeAppLocalNode({
   if (!spawnedProcess.pid) {
     // Attempt to capture any immediate errors if possible
     let errorOutput = "";
-    spawnedProcess.stderr?.on("data", (data) => (errorOutput += data));
-    await new Promise((resolve) => spawnedProcess.on("error", resolve)); // Wait for error event
-    throw new Error(
-      `Failed to spawn process for app ${appId}. Error: ${
-        errorOutput || "Unknown spawn error"
+    let spawnErr: any | null = null;
+    spawnedProcess.stderr?.on(
+      "data",
+      (data) => (errorOutput += data.toString()),
+    );
+    await new Promise<void>((resolve) => {
+      spawnedProcess.once("error", (err) => {
+        spawnErr = err;
+        resolve();
+      });
+    }); // Wait for error event
+
+    const details = [
+      spawnErr?.message ? `message=${spawnErr.message}` : null,
+      spawnErr?.code ? `code=${spawnErr.code}` : null,
+      spawnErr?.errno ? `errno=${spawnErr.errno}` : null,
+      spawnErr?.syscall ? `syscall=${spawnErr.syscall}` : null,
+      spawnErr?.path ? `path=${spawnErr.path}` : null,
+      spawnErr?.spawnargs
+        ? `spawnargs=${JSON.stringify(spawnErr.spawnargs)}`
+        : null,
+    ]
+      .filter(Boolean)
+      .join(", ");
+
+    logger.error(
+      `Failed to spawn process for app ${appId}. Command="${command}", CWD="${appPath}", ${details}\nSTDERR:\n${
+        errorOutput || "(empty)"
       }`,
     );
+
+    throw new Error(
+      `Failed to spawn process for app ${appId}.
+Error output:
+${errorOutput || "(empty)"}
+Details: ${details || "n/a"}
+`,
+    );
   }
 
   // Increment the counter and store the process reference with its ID
@@ -409,13 +440,40 @@ RUN npm install -g pnpm
   if (!process.pid) {
     // Attempt to capture any immediate errors if possible
     let errorOutput = "";
-    process.stderr?.on("data", (data) => (errorOutput += data));
-    await new Promise((resolve) => process.on("error", resolve)); // Wait for error event
-    throw new Error(
-      `Failed to spawn Docker container for app ${appId}. Error: ${
-        errorOutput || "Unknown spawn error"
+    let spawnErr: any = null;
+    process.stderr?.on("data", (data) => (errorOutput += data.toString()));
+    await new Promise<void>((resolve) => {
+      process.once("error", (err) => {
+        spawnErr = err;
+        resolve();
+      });
+    }); // Wait for error event
+
+    const details = [
+      spawnErr?.message ? `message=${spawnErr.message}` : null,
+      spawnErr?.code ? `code=${spawnErr.code}` : null,
+      spawnErr?.errno ? `errno=${spawnErr.errno}` : null,
+      spawnErr?.syscall ? `syscall=${spawnErr.syscall}` : null,
+      spawnErr?.path ? `path=${spawnErr.path}` : null,
+      spawnErr?.spawnargs
+        ? `spawnargs=${JSON.stringify(spawnErr.spawnargs)}`
+        : null,
+    ]
+      .filter(Boolean)
+      .join(", ");
+
+    logger.error(
+      `Failed to spawn Docker container for app ${appId}. ${details}\nSTDERR:\n${
+        errorOutput || "(empty)"
       }`,
     );
+
+    throw new Error(
+      `Failed to spawn Docker container for app ${appId}.
+Details: ${details || "n/a"}
+STDERR:
+${errorOutput || "(empty)"}`,
+    );
   }
 
   // Increment the counter and store the process reference with its ID