(core) Improve debug logging related for client-side errors and sandbox crashes.

Summary:
- Include docId when available for client-side error reporting
- Distinguish sandbox crashes from forced exits

Test Plan: Tested manually

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3373

app/server/lib/FlexServer.ts

@@ -576,6 +576,7 @@ export class FlexServer implements GristServer {
       const mreq = req as RequestWithLogin;
       log.rawWarn('client error', {
         event: req.body.event,
+        docId: req.body.docId,
         page: req.body.page,
         browser: req.body.browser,
         org: mreq.org,

app/server/lib/NSandbox.ts

@@ -233,8 +233,13 @@ export class NSandbox implements ISandbox {
   }
 
   private _onExit(code: number, signal: string) {
+    const expected = this._isWriteClosed;
     this._close();
-    log.rawDebug(`Sandbox exited with code ${code} signal ${signal}`, this._logMeta);
+    if (expected) {
+      log.rawDebug(`Sandbox exited with code ${code} signal ${signal}`, this._logMeta);
+    } else {
+      log.rawWarn(`Sandbox unexpectedly exited with code ${code} signal ${signal}`, this._logMeta);
+    }
   }