Improved error message

[codecov[bot]]

Codecov Report

Base: 87.70% // Head: 87.69% // Decreases project coverage by -0.01% :warning:

Coverage data is based on head (27dff8f) compared to base (7d8ccdd). Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #320 +/- ## ========================================== - Coverage 87.70% 87.69% -0.02% ========================================== Files 69 69 Lines 10381 10382 +1 ========================================== - Hits 9105 9104 -1 - Misses 1276 1278 +2 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `87.69% <50.00%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/zwicker-group/py-pde/pull/320?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group) | Coverage Δ | | |---|---|---| | [pde/grids/\_mesh.py](https://codecov.io/gh/zwicker-group/py-pde/pull/320/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL2dyaWRzL19tZXNoLnB5) | `94.58% <50.00%> (-0.35%)` | :arrow_down: | | [pde/grids/base.py](https://codecov.io/gh/zwicker-group/py-pde/pull/320/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL2dyaWRzL2Jhc2UucHk=) | `89.14% <0.00%> (-0.17%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group)

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