feat[ux]: improve error message for importing ERC20

[charles-cooper]

improve error message for common issue when upgrading from v0.3.x

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Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 85.09%. Comparing base (b53a020) to head (8a6f844).

Files	Patch %	Lines
vyper/semantics/analysis/module.py	0.00%	8 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3816      +/-   ##
==========================================
- Coverage   85.15%   85.09%   -0.07%     
==========================================
  Files          92       92              
  Lines       13907    13907              
  Branches     3116     3116              
==========================================
- Hits        11843    11834       -9     
- Misses       1571     1581      +10     
+ Partials      493      492       -1     

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