BUG: remove NotImplementedError from traceback

[Miksus]

Remove the annoying NotImplementedError from the traceback.

[codecov-commenter]

Codecov Report

Base: 91.96% // Head: 92.01% // Increases project coverage by +0.04% :tada:

Coverage data is based on head (ccf8250) compared to base (c76a5e4). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #45 +/- ## ========================================== + Coverage 91.96% 92.01% +0.04% ========================================== Files 22 22 Lines 1419 1427 +8 ========================================== + Hits 1305 1313 +8 Misses 114 114 ``` | [Impacted Files](https://codecov.io/gh/Miksus/red-bird/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mikael+Koli) | Coverage Δ | | |---|---|---| | [redbird/templates.py](https://codecov.io/gh/Miksus/red-bird/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mikael+Koli#diff-cmVkYmlyZC90ZW1wbGF0ZXMucHk=) | `97.56% <100.00%> (+0.26%)` | :arrow_up: | 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=Mikael+Koli). 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=Mikael+Koli)

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