(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,