Fix DtypeError message for reference validation

[stephprince]

Motivation

Fix #1055. When validating a reference, the RefSpec input to the DtypeError message initialization was causing an inaccurate error message.

How to test the behavior?

See example file shared in the original matnwb issue.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.08%. Comparing base (6e5534c) to head (2253f0e).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1199      +/-   ##
==========================================
+ Coverage   85.62%   89.08%   +3.46%     
==========================================
  Files          45       45              
  Lines        9887     9890       +3     
  Branches     2815     2816       +1     
==========================================
+ Hits         8466     8811     +345     
+ Misses       1076      763     -313     
+ Partials      345      316      -29     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[rly]

Looks good. Thank you @stephprince !