Fix nexus save

[samtygier-stfc]

Issue

Closes #2404

Description

Async tasks need to return something, because the None looks like a failed task.
Add a test so that this would have been caught.
Make the test_save_images able to catch the equivalent issue
Make load and save fails raise an exception rather than just open a dialog, so that problems are easier for the tests to catch. This means the test_failed_attempt_to_load_shows_error and test_failed_attempt_to_save_shows_error tests now test for an exception.

Testing

A new test and updates to others

Acceptance Criteria

Check that you can save a nexus file without errors.
Remove the return True from do_nexus_saving() and check that you still get an error dialog and the the system tests (make test-system) catch the problem.

Documentation

Release notes

[coveralls]

coverage: 74.222% (-0.005%) from 74.227%
when pulling 9dd000d on 2402-fix-nexus-save
into e027cbb on main.

[JackEAllen]

Thanks for resolving this, happy to approve. The changes to do_nexus_saving() and the addition of a new system test look good. I have tested saving as nexus and also followed your instructions to check the system test passes and fails as expected under the right conditions.