500 Internal Server Error when POST reply via /:channel/content/posts with invalid ID

[scholary-tim]

Replication Steps:
Send a message to the API server that posts a reply to message with an invalid ID.

Headers: Basic Authentication [USERNAME/PASSWORD]
Body:

<entry xmlns="http://www.w3.org/2005/Atom">
  <in-reply-to ref="this-should-cause-a-problem123" xmlns="http://purl.org/syndication/thread/1.0"/>
  <content>A lovely little reply via the RESTClient (in FF 21)</content>
</entry>

Expected Outcome:
400 (Bad Request). Server handles response nicely.

Actual Outcome:
500 (Internal Server Error).

I suspect server logs may throw something useful, but how would the client handle this (edge case) scenario? Raised as an enhancement - is there a nicer way to handle this?