Include stack trace in overflow queue failure message

Suggested release note: N/A (primarily for internal diagnostics)

A user is seeing this message printed to stderr:

Encountered an error while processing the overflow queue - this is a bug: java.lang.NullPointerException

Something is going wrong with the overflow queue processing, but we don't know where the NPE comes from because the stack trace is not logged.

Let's include the stack trace in the message!