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[bot]]

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 6 commits behind head on master.

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

Additional details and impacted files

```diff @@ 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.