Improve test coverage of the `betty.gramps.loader` module

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.95918% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.82%. Comparing base (b5a9cdd) to head (cbe3c9a). Report is 1 commits behind head on 0.4.x.

Files with missing lines	Patch %	Lines
betty/gramps/loader.py	96.15%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.4.x #1913 +/- ## ========================================== + Coverage 96.73% 96.82% +0.09% ========================================== Files 259 259 Lines 18263 18310 +47 Branches 3463 3475 +12 ========================================== + Hits 17666 17729 +63 + Misses 405 392 -13 + Partials 192 189 -3 ```

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