If record has an err set the stack to message for Stackdriver display

Stackdriver/Google error reporting requires that error logs have a V8 stacktrace as the payload `message`: https://cloud.google.com/error-reporting/docs/formatting-error-messages

They also require setting a service context (`"serviceContext": {"service": "worker"}`) but this can be done in serialisers or on Bunyan loggers themselves

Author: Binarytales

lib/bunyan-stackdriver.js

@@ -93,7 +93,9 @@ BunyanStackDriver.prototype._write = function write(record, encoding, callback)
   record = destroyCircular(record);
   strictJSON(record);
 
-  if (!record.message) {
+  if (record.err) {
+      record.message = record.err.stack;
+  } else if (!record.message) {
     record.message = record.msg;
   }