Revised exception handling and adjustments suggested by Markus

gui/src/components/api.js

nomad/app/v1/models.py

nomad/app/v1/routers/entries.py

nomad/app/v1/routers/uploads.py

nomad/cli/client/integrationtests.py

nomad/processing/data.py

tests/app/v1/routers/test_entries_edit.py

tests/app/v1/routers/test_uploads.py

tests/processing/test_edit_metadata.py