[FIX] google_calendar: improve error handling when the record does not exists anymore.

Before this commit, the following traceback could be encountered when events were deleted:

```py
Traceback (most recent call last):
  File "/home/odoo/src/odoo/15.0/addons/google_calendar/models/google_sync.py", line 43, in called_after
    func(self.with_env(env), *args, **kwargs)
  File "/home/odoo/src/odoo/15.0/addons/google_calendar/models/google_sync.py", line 233, in _google_patch
    self._google_error_handling(e)
  File "/home/odoo/src/odoo/15.0/addons/google_calendar/models/google_sync.py", line 177, in _google_error_handling
    start = self.start and self.start.strftime('%Y-%m-%d at %H:%M') or _("undefined time")
  File "/home/odoo/src/odoo/15.0/odoo/fields.py", line 1087, in __get__
    raise MissingError("\n".join([
odoo.exceptions.MissingError: Record does not exist or has been deleted.
(Record: calendar.event(28241777,), User: XXXX)
```

X-original-commit: e77cbc639032e7f58345942330102b37e7a55971
Part-of: odoo/odoo#83040

addons/google_calendar/models/google_sync.py

@@ -173,6 +173,12 @@ class GoogleSync(models.AbstractModel):
         # We only handle the most problematic errors of sync events.
         if http_error.response.status_code in (403, 400):
             response = http_error.response.json()
+            if not self.exists():
+                reason = "Google gave the following explanation: %s" % response['error'].get('message')
+                error_log = "Error while syncing record. It does not exists anymore in the database. %s" % reason
+                _logger.error(error_log)
+                return
+
             if self._name == 'calendar.event':
                 start = self.start and self.start.strftime('%Y-%m-%d at %H:%M') or _("undefined time")
                 event_ids = self.id