tps-frontend: "Failed to set text" when formatting markup in error dialog

The error message in the description of #20163 contains:

Jan 21 15:07:35 amnesia python3[8681]: Failed to set text '<span insert-hyphens='no'>gi.repository.GLib.Error: udisks-error-quark: GDBus.Error:org.freedesktop.UDisks2.Error.Failed: Error creating partition on /dev/sdb: Process reported exit code 2:
Warning! Main and backup partition tables differ! Use the 'c' and 'e' options
				       on the recovery & transformation menu to examine the two tables.
[...]

The Failed to set text part it printed by Gtk.MessageDialog.format_secondary_markup() which is called in https://gitlab.tails.boum.org/tails/tails/-/blob/e154eddea9e83bb4273fb4cd1aa86e81b015eeb7/config/chroot_local-includes/usr/lib/python3/dist-packages/tps_frontend/error_dialog.py#L57-57.

As a result, an empty error message like this is shown: