Database errors should cause an XMPP error

[guusdk]

When there's an issue interacting with the database while retrieving messages, those issues are currently ignored. This leads to results that are incomplete.

Fail-fast behavior is desired here: when there's a database error, thrown an exception.

It's probably best to make this configurable, to allow administrators to fall back to ignoring errors, for those instances where, for a reason that's currently beyond me, have significant amounts of database issues.

[guusdk]

The property that can be used to control this behavior is archive.ignore-retrieval-exceptions. It's value is a boolean. false, which will be the new default, will cause XMPP errors to be generated, while true restores the old behavior of ignoring errors.