Fix error message after data.table change

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

data.table changed an error message that we were checking for in tests. This PR updates the message we're checking for.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Columbia University

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.14%. Comparing base (9fcbbd0) to head (36d76f1).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1127      +/-   ##
==========================================
- Coverage   87.15%   87.14%   -0.02%     
==========================================
  Files          14       14              
  Lines        5973     5973              
==========================================
- Hits         5206     5205       -1     
- Misses        767      768       +1     

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