Describe the bug
There is code in queue_outbound_respond and queue_respond that assumes msg is a string, when passed a DSN object it ends up using returning "xxx [object Object] (uuid)" type strings.
When msg is an array I think you're going to have problems, too.

The errant bit of code code looks like this:
${msg} (${this.transaction.uuid})
Inside an interpolated string.

It's helpfully adding the uuid to the message string. Except if the msg is not a string but a DSN object.
This kind of error is much harder to make when using TypeScript.

Expected behavior
I expect the custom DSN objects using the haraka-dsn plugin should work the same for each hook.

Observed behavior
The message gets "[object Object]" in place of all that info in the DSN.

Steps To Reproduce
Create a plugin that uses hook_queue or hook_queue_outbound, call next(DENY, new DSN(...)); and observe the returned SMTP error message.

System Info:
This node 14 I think on Linux. The Haraka version is the head of the git repo.

Additional context
I think it would be enough to just check if typeof msg === 'string' or whatever before appending the uuid.