Fix E_loo_khat error when posterior::pareto_khat returns NA

[jgabry]

closes #263

This avoids the error mentioned by @bgoodri in #263 by first checking if posterior::pareto_khat returns a list or NA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.69%. Comparing base (1ccfc44) to head (afe9bb5).

❗ Current head afe9bb5 differs from pull request most recent head bd4e7f8. Consider uploading reports for the commit bd4e7f8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   92.68%   92.69%   +0.01%     
==========================================
  Files          31       31              
  Lines        2842     2846       +4     
==========================================
+ Hits         2634     2638       +4     
  Misses        208      208              

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[avehtari]

Doesn't look nice, but I don't see how else to do it