fix: Ensure Terminating Thread error is correctly templatized (#738)

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({}, [], []); ```

runner/src/stream-handler/stream-handler.ts

@@ -67,8 +67,8 @@ export default class StreamHandler {
     indexer.setStoppedStatus().catch((e) => {
       this.logger.error('Failed to set stopped status for indexer', e);
     });
-
-    const streamErrorLogEntry = LogEntry.systemError(`Encountered error processing stream: ${this.indexerConfig.redisStreamKey}, terminating thread\n${error.toString()}`, this.executorContext.block_height);
+    const errorContent = error instanceof Error ? error.toString() : JSON.stringify(error);
+    const streamErrorLogEntry = LogEntry.systemError(`Encountered error processing stream: ${this.indexerConfig.redisStreamKey}, terminating thread\n${errorContent}`, this.executorContext.block_height);
 
     indexer.writeCrashedWorkerLog(streamErrorLogEntry)
       .catch((e) => {