Make map_src public on error types

[jswrenn]

Closes #1297

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 87.91%. Comparing base (70f3e14) to head (d85c48e).

Files with missing lines	Patch %	Lines
src/error.rs	40.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1686      +/-   ##
==========================================
- Coverage   88.01%   87.91%   -0.10%     
==========================================
  Files          16       16              
  Lines        5805     5811       +6     
==========================================
  Hits         5109     5109              
- Misses        696      702       +6     

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