added error message for priors

[codecov[bot]]

Codecov Report

Base: 49.94% // Head: 49.88% // Decreases project coverage by -0.05% :warning:

Coverage data is based on head (3e7511c) compared to base (5d2c0a6). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #116 +/- ## ========================================== - Coverage 49.94% 49.88% -0.06% ========================================== Files 23 23 Lines 2625 2628 +3 Branches 498 499 +1 ========================================== Hits 1311 1311 - Misses 1198 1201 +3 Partials 116 116 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `49.88% <0.00%> (-0.06%)` | :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=Andrei+Cuceu#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/andreicuceu/vega/pull/116?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrei+Cuceu) | Coverage Δ | | |---|---|---| | [vega/vega\_interface.py](https://codecov.io/gh/andreicuceu/vega/pull/116/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrei+Cuceu#diff-dmVnYS92ZWdhX2ludGVyZmFjZS5weQ==) | `58.71% <0.00%> (-0.68%)` | :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=Andrei+Cuceu). 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=Andrei+Cuceu)

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