On export, skip encrypted submissions with manifest but no .enc file and mark as failed

[lognaturel]

Software versions

Briefcase v1.18 and prior

Problem description

Submissions to encrypted forms can be missing the submission file itself. In that case, export crashes.

See https://forum.getodk.org/t/odk-briefcase-1-18-export-failing-with-encrypted-forms/31069

Steps to reproduce the problem

Set up an encrypted form with submissions. Delete the .enc file for some submissions. Try to export.

Expected behavior

Export should not crash. Incomplete submissions should be marked as problematic.

Other information

Similar to https://github.com/getodk/briefcase/pull/884