Comment 6 for bug 1614576

For future reference one annoying thing about the non-interactive version of the upgrader is that it has its own exception handler e.g.:

class DistUpgradeViewNonInteractive(DistUpgradeView):
    " non-interactive version of the upgrade view "
    def __init__(self, datadir=None, logdir=None):
        DistUpgradeView.__init__(self)
        self.config = DistUpgradeConfig(".")
        self._acquireProgress = NonInteractiveAcquireProgress()
        self._installProgress = NonInteractiveInstallProgress(logdir)
        self._opProgress = apt.progress.base.OpProgress()
        sys.__excepthook__ = self.excepthook

Disabling sys.__excepthook__ allowed the real error, UnicodeDecodeError, to surface.