chore[ux]: improve an exports error message

[charles-cooper]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.19%. Comparing base (391f3cc) to head (5615f1c).
Report is 1 commits behind head on master.

Files	Patch %	Lines
vyper/semantics/analysis/module.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3822      +/-   ##
==========================================
+ Coverage   85.18%   85.19%   +0.01%     
==========================================
  Files          92       92              
  Lines       13908    13908              
  Branches     3116     3116              
==========================================
+ Hits        11847    11849       +2     
+ Misses       1567     1565       -2     
  Partials      494      494              

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