Exit code 2 if TreeTimeError encountered

[jameshadfield]

Current behaviour is to exit code 0 ("success") if augur encounters TreeTimeError. The initial implementation
(3256319) exited code 2 but this was removed by 403da88

This bug was first observed (by me, at least) as part of #1360

• Checks pass. The relevant checks pass, but CI is failing due to CI failure: nextclade3 command not found rsv#50
• If making user-facing changes, add a message in CHANGES.md summarizing the changes in this PR

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e346693) 66.14% compared to head (e98f088) 66.13%.

Files	Patch %	Lines
augur/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1367      +/-   ##
==========================================
- Coverage   66.14%   66.13%   -0.01%     
==========================================
  Files          68       68              
  Lines        7221     7222       +1     
  Branches     1777     1777              
==========================================
  Hits         4776     4776              
- Misses       2176     2177       +1     
  Partials      269      269              

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