Improved ancestral translation validation

[corneliusroemer]

Description of proposed changes

Wasted some time trying to figure things out that would have been helped by better validation. So thought I'll give it a shot.

• Checks pass
• If making user-facing changes, add a message in CHANGES.md summarizing the changes in this PR

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 65.74%. Comparing base (2ec73cd) to head (7b564b7).
Report is 614 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/ancestral.py	41.66%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1321      +/-   ##
==========================================
- Coverage   65.79%   65.74%   -0.05%     
==========================================
  Files          68       68              
  Lines        7151     7161      +10     
  Branches     1757     1762       +5     
==========================================
+ Hits         4705     4708       +3     
- Misses       2182     2185       +3     
- Partials      264      268       +4     

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[corneliusroemer]

Don't really think this is that useful