chore(ancestral): more informative AugurError message

[corneliusroemer]

Description of proposed changes

Related issue(s)

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.27%. Comparing base (efabf11) to head (c5e74b3).
Report is 160 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/ancestral.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1662   +/-   ##
=======================================
  Coverage   72.26%   72.27%           
=======================================
  Files          79       79           
  Lines        8271     8273    +2     
  Branches     1691     1691           
=======================================
+ Hits         5977     5979    +2     
  Misses       2009     2009           
  Partials      285      285           

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[genehack]

Unsure if updating an error message constitutes a "functional change" that would require a Changelog entry…

[corneliusroemer]

Thanks! It's such a small change, I won't pollute the changelog with it.