Improve titer model error messages

[huddlej]

Description of proposed changes

Replaces the generic "not enough data" error message with the more detailed messages associated with the InsufficientDataException when it is raised.

Related issue(s)

Related to nextstrain/seasonal-flu#215

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 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.22%. Comparing base (8cf91ae) to head (c21c5a8).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/titers.py	0.00%	4 Missing ⚠️
augur/titer_model.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1769   +/-   ##
=======================================
  Coverage   73.22%   73.22%           
=======================================
  Files          79       79           
  Lines        8391     8391           
  Branches     1712     1712           
=======================================
  Hits         6144     6144           
  Misses       1959     1959           
  Partials      288      288           

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