Improve error logs for classifier operations + fix complex tests

[vincentschen]

Description of proposed changes

• Show the underlying exception for failed Operations
• Fix complex tests by casting inputs to float32 (rather than doubles)

Test plan

• Ran tox, tox -e complex, tox -e spark locally; passed

Checklist

• I have read the CONTRIBUTING document.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have run tox -e complex and/or tox -e spark if appropriate.
• All new and existing tests passed.

[henryre]

Looks good, let's track to make sure Travis passes on master

[codecov]

Codecov Report

Merging #1597 into master will increase coverage by 0.06%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #1597      +/-   ##
==========================================
+ Coverage   97.13%   97.19%   +0.06%     
==========================================
  Files          56       68      +12     
  Lines        2091     2137      +46     
  Branches      342      343       +1     
==========================================
+ Hits         2031     2077      +46     
  Misses         31       31              
  Partials       29       29              

Impacted Files	Coverage Δ
...el/classification/training/loggers/checkpointer.py	96.90% <ø> (ø)
snorkel/labeling/lf/core.py	100.00% <ø> (ø)
snorkel/preprocess/core.py	100.00% <ø> (ø)
snorkel/slicing/sf/nlp.py	100.00% <ø> (ø)
snorkel/classification/training/trainer.py	89.83% <50.00%> (ø)
snorkel/labeling/model/label_model.py	95.54% <50.00%> (ø)
snorkel/classification/multitask_classifier.py	96.53% <100.00%> (ø)
snorkel/labeling/lf/nlp.py	100.00% <100.00%> (ø)
snorkel/map/core.py	100.00% <100.00%> (ø)
snorkel/preprocess/nlp.py	86.66% <100.00%> (ø)
... and 15 more