[FIX] Predictions: Fix failure after failed predictor

[janezd]

Issue

I predictor fails, it sets results to a string describing an error. When _call_predictors is called again, it tests for pred.results is None or numpy.isnan(pred.results[0]).all(), which crashes because pred.results is a string.

Description of changes

Test for isinstance(pred.results, str) and treat it as None or all nans.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #3337 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3337      +/-   ##
==========================================
+ Coverage   82.21%   82.27%   +0.06%     
==========================================
  Files         351      351              
  Lines       62301    62575     +274     
==========================================
+ Hits        51219    51485     +266     
- Misses      11082    11090       +8