catch Stan log_prob exceptions inside moment matching

[avehtari]

Fixes #261

When doing moment matching steps, catch the errors when evaluating log_prob, and break out without an error. This will allow getting moment matching results for all the non-failing observations and may get some moment matching also for the failing ones.

[jgabry]

This seems like a good idea. I made a few small suggestions in review comments.

[avehtari]

Thanks for fixes! I need to remember to run tests in fresh R session. I was learning try and TryCatch and for a moment thought I might need the latter, and for some reason didn't see that it was still there

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 92.68%. Comparing base (52bc270) to head (f2ea5d4).

❗ Current head f2ea5d4 differs from pull request most recent head 36ffa4d. Consider uploading reports for the commit 36ffa4d to get more accurate results

Files	Patch %	Lines
R/loo_moment_matching.R	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
- Coverage   92.77%   92.68%   -0.10%     
==========================================
  Files          31       31              
  Lines        2836     2842       +6     
==========================================
+ Hits         2631     2634       +3     
- Misses        205      208       +3     

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