fix: Ensure Terminating Thread error is correctly templatized

[darunrs]

The error given to Stream Handler is occasionally a JSON object in disguise, and not an Error. As a result, calling error.toString() returns [Object object] rather than the error contents. I've added a check so that if the result of toString() is the earlier value, return a JSON.stringify result instead. Doing JSON.stringify on a proper Error type results in {}. The two cases must be handled separately.

To test this, I created test indexers which called one of the two pieces of code which throw unawaited async errors. I verified both wrote the error message and stack trace into the log table.

const timeoutPromise = new Promise((_, reject) => {
  setTimeout(() => {
      reject(new Error('Error thrown after 100ms'));
  }, 100);
});

context.db.IndexerStorage.upsert({}, [], []);

[morgsmccauley]

Suggested change

	const errorAsString = error.toString() === '[object Object]' ? JSON.stringify(error) : error.toString();
	const errorMessage = error instanceof Error ? error.toString() : JSON.stringify(error);

I believe [Object object] would only appear if we are throwing an Object somewhere. I think we can just change this to the above to ensure we are only calling toString() on Errors.

[darunrs]

Tested this and it seems to work. I think this is cleaner so I'll go with this. Thanks!