Improve pie chart error messages

[costaparas]

PR summary

Fix typo in error message, add more detail and make formatting consistent

PR checklist

• [na] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [na] Plotting related features are demonstrated in an example
• [na] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

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[rcomer]

Thanks @costaparas! I agree this is better.

[story645]

Thanks for the PR!

Code coverage error is related - these lines aren't tested, but I think codecover is only picking them up now b/c they haven't been changed since we introduced code cover.

Would you be interested in writing tests to check that these error messages get triggered?

[costaparas]

Thanks for the PR!

Code coverage error is related - these lines aren't tested, but I think codecover is only picking them up now b/c they haven't been changed since we introduced code cover.

Would you be interested in writing tests to check that these error messages get triggered?

Yes, added. Thanks.

[story645]

Congrats on your first merged PR, thanks for also putting in the tests, and we hope to see you again!