(core) Clear error.__traceback__ to prevent memory leaks

Summary: When a formula raises an exception, we store that in the cell in memory. In Python 3, exceptions have a `__traceback__` attribute, which includes all the stack frames and local variables. This has huge memory leak potential. We already strategically format the exception when needed, we don't need to keep storing the actual traceback object.

Test Plan:
Manually tested that tracebacks are still sensible.

To check the effect on memory usage, made a simple test doc with 30k rows all containing an exception, and here's what ps aux says:

```
       %MEM    VSZ   RSS
before: 2.4 681996 588828
after:  1.6 499052 405712
```

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3505

sandbox/grist/objtypes.py

@@ -275,6 +275,7 @@ class RaisedException(object):
     self._message = None
     if error is not None:
       self._fill_from_error(self.has_user_input(), include_details)
+      error.__traceback__ = None
 
   def encode_args(self):
     if self._encoded_error is not None: