Fix/skyline export

[rneher]

Description of proposed changes

this catches a failed skyline optimization by treetime and fails refine.

Related issue(s)

Related to #554

Testing

This seems to have happened in some edge cases and has its root in failure of convergence of the skyline optimziation

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[codecov]

Codecov Report

Merging #556 into master will decrease coverage by 0.13%.
The diff coverage is 8.79%.

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
- Coverage   19.47%   19.33%   -0.14%     
==========================================
  Files          31       31              
  Lines        5073     5120      +47     
  Branches     1284     1301      +17     
==========================================
+ Hits          988      990       +2     
- Misses       4062     4106      +44     
- Partials       23       24       +1     

Impacted Files	Coverage Δ
augur/refine.py	5.32% <0.00%> (-0.13%)	⬇️
augur/traits.py	7.82% <0.00%> (ø)
augur/utils.py	27.63% <0.00%> (ø)
augur/validate.py	25.80% <0.00%> (ø)
augur/parse.py	10.60% <3.22%> (ø)
augur/align.py	38.46% <14.28%> (-8.17%)	⬇️

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