fix column coerce warning, decorator Schema error failure cases

[cosmicBboy]

failure_cases wasn't correctly being set in the decorator functions

[codecov-commenter]

Codecov Report

Merging #228 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   96.52%   96.67%   +0.15%     
==========================================
  Files          14       15       +1     
  Lines        1236     1413     +177     
==========================================
+ Hits         1193     1366     +173     
- Misses         43       47       +4     

Impacted Files	Coverage Δ
pandera/decorators.py	91.37% <100.00%> (+0.30%)	⬆️
pandera/schemas.py	96.96% <100.00%> (+0.36%)	⬆️
pandera/__init__.py	100.00% <0.00%> (ø)
pandera/io.py	91.16% <0.00%> (+1.01%)	⬆️

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