webhooks: Do log non-"unsupported" errors to main logfiles.

This undoes a small part of b8a2e6b5f8; namely, logs to
`zulip.zerver.webhooks`, which are all exceptions from webhooks except
UnsupportedWebhookEventType, should still be logged to the main error
loggers.  This maintains the property that exceptions generating 500's
are all present in `errors.log`.

zproject/computed_settings.py

@@ -952,7 +952,7 @@ LOGGING: Dict[str, Any] = {
         },
         'zulip.zerver.webhooks': {
             'level': 'DEBUG',
-            'handlers': ['webhook_file'],
+            'handlers': ['file', 'errors_file', 'webhook_file'],
             'propagate': False,
         },
         'zulip.zerver.webhooks.unsupported': {