Summary: Capture the stacktrace (via SandboxError) in `_pyCallWait` instead of `_onSandboxMsg` where it's always the same.
Test Plan:
Tested manually, found for example that the stacktrace in the logs changed from being rather useless:
```
at NSandbox._onSandboxMsg (/home/alex/work/grist/_build/core/app/server/lib/NSandbox.js:229:36)
at /home/alex/work/grist/_build/core/app/server/lib/NSandbox.js:179:18
at Unmarshaller.parse (/home/alex/work/grist/_build/core/app/common/marshal.js:289:21)
at NSandbox._onSandboxData (/home/alex/work/grist/_build/core/app/server/lib/NSandbox.js:174:28)
at Socket.<anonymous> (/home/alex/work/grist/_build/core/app/server/lib/NSandbox.js:63:59)
at Socket.emit (events.js:315:20)
at Socket.EventEmitter.emit (domain.js:467:12)
at addChunk (internal/streams/readable.js:309:12)
at readableAddChunk (internal/streams/readable.js:284:9)
at Socket.Readable.push (internal/streams/readable.js:223:10)
at Pipe.onStreamRead (internal/stream_base_commons.js:188:23)
```
to being somewhat more helpful:
```
at NSandbox._pyCallWait (/home/alex/work/grist/_build/core/app/server/lib/NSandbox.js:134:19)
at processTicksAndRejections (internal/process/task_queues.js:93:5)
at async ActiveDoc.applyActionsToDataEngine (/home/alex/work/grist/_build/core/app/server/lib/ActiveDoc.js:1080:39)
at async Sharing._applyActionsToDataEngine (/home/alex/work/grist/_build/core/app/server/lib/Sharing.js:325:37)
```
Reviewers: paulfitz
Reviewed By: paulfitz
Subscribers: paulfitz
Differential Revision: https://phab.getgrist.com/D3329
1452b6efc3