Make restore_items() consistent with remove_items()

[soininen]

DatabaseMapping.restore_items() now returns restored items and errors like DatabaseMapping.remove_items() does.

Re spine-tools/Spine-Toolbox#2861

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.29%. Comparing base (7422f1b) to head (b723ce8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #424 +/- ## ========================================== + Coverage 78.27% 78.29% +0.02% ========================================== Files 72 72 Lines 9577 9575 -2 Branches 2045 2044 -1 ========================================== + Hits 7496 7497 +1 + Misses 1741 1738 -3 Partials 340 340 ```

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